A well-made roast beef sandwich can be magical. A perfectly rare roast beef sandwich is a thing of beauty.
My take on the classic roast beef sandwich features perfectly cooked rare roast beef, juicy slow-roasted tomatoes, and sweet sauteed onions.
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Roast Beef Makes a Great Picnic Sandwich
In the world of picnic sandwiches, the roast beef sandwich is high in the pecking order of meat sandwiches—up there with the best chicken sandwiches, of course.
There are so many versions of the classic roast beef sandwich, whether using leftover beef from a Sunday roast, opting for the ease of deli-bought slices, or roasting a loin solely for the purpose (I’ll give you tips for this later).
Roast beef makes a good hearty sandwich for any season but is especially suitable for cooler-weather outings such as winter picnics. I love a simple picnic with roast beef sandwiches and a delicious thermos of picnic soup – it’s so simple yet hearty and delicious.
Sandwich Ingredients
See my recipe card below for a complete list of the ingredients with measurements and notes.
Recipe Variations and Substitutions
- Salad Greens: I use watercress because its peppery flavor complements the beef. You can use whichever green you like. Baby spinach or arugula are good options for a roast beef sandwich.
- Mustard: I like seeded mustard with roast beef, but you can use any mustard you prefer. Horseradish cream is also a good choice if you like some heat.
- Cheese: I don’t add cheese to this sandwich because I don’t feel it needs it. Cheddar, provolone, Swiss, and blue cheese are all good options for this sandwich if you want to add cheese.
- Balsamic Vinegar: I suggest adding balsamic vinegar when sauteing the onions, but this is optional. I like the sweet, stickiness balsamic brings to the onions, but you can leave it out.
Step by Step Instructions
Step 1: Place the thinly sliced onion in a frying pan with a small dob of butter and a drizzle of olive oil. Cook on medium heat, stirring often, being careful to sweat the onions, not burn them. Cook until the onions are soft and caramelized.
Optional: As the onions soften, add a small drizzle of balsamic vinegar and continue to stir (the balsamic will help the onions become a nice, sweet, jam-like consistency).
Step 2: Spread the whole grain mustard on one slice of the bread, layer with slices of roast beef, and top with the caramelized onion.
Step 3: Top with greens of your choice ( I use watercress) and roasted tomato halves (See my recipe for Roasted Vine Tomatoes for tips).
Step 4: Drizzle with any of the tomato juice before topping with the other slice of bread.
Recipe FAQs
While using last night’s leftover rib roast is perfectly fine, this delicious bottom-round roast recipe is cooked slowly and reverse-seared. It’s a great alternative oven roast and perfect for roast beef sandwiches.
Deli’s, nuanced in the art of the roast beef sandwich, opt for a loin roast or eye round. These cuts are lean, with fat on the top, as opposed to the marbling that makes your Sunday roast juicy and tender straight from the oven.
The marbled fat can solidify when cold, creating tough veins in your slices. As a rule of thumb, these leaner cuts will be more tender and juicy when served cold. You can read more about the best cut of meat for a roast beef sandwich here.
I often roast a loin for this purpose. It makes a great meal served hot, and the perfectly cooked beef is ready to slice the next day for your picnic sandwiches.
I cook the beef over the coals where possible to get a nice crispy charred outer and a touch of smokiness. But cooking it in the oven is fine and will have the same internal temperature for a perfect rare finish. 
Using an inexpensive meat thermometer will help you achieve the perfect doneness for your roast beef sandwich. The following is a guide for the internal temperature of roast beef doneness. These are the out-of-oven temperatures. Your beef should be left to rest after cooking. The meat will continue to cook (the internal temperature should rise). Resting also prevents the meat from drying out.
Roast Beef Cooking Temperatures
Rare – 50°C |122°F
Medium rare – 55°C |131°F
Medium – 60°C |140°F
Medium well – 65°C |149°F
Well done – 70°C |158°F
One of my key picnic tips for sandwiches is always to go with bread and fillings that will hold their own without going soggy. Slices of soft white bread are fine for a simple ham and cheese and a match made in heaven for cucumber sandwiches. But a bold sandwich such as roast beef, especially with the addition of slow-roasted tomatoes, needs sturdy bread to avoid soggy sandwich syndrome.
In this case, I have gone with crusty slices of homemade sourdough. Rye bread, baguettes, bagels, and ciabattas are strong candidates for a successful roast beef picnic sandwich. Bread that will soak up the flavors perfectly without becoming soggy.
Expert Tips
Thin Slices of Beef: Thin slices are key for a tender sandwich that won’t fall apart when you bite into it. If you have an electric slicer, use it for uniform, thin slices. Otherwise, let the roast beef chill in the refrigerator before slicing it with a sharp knife, as cold beef is easier to slice thinly.
Toast the Bread: If you want to add some crunch to your sandwich, you can toast the bread. I find toasting only the outside of the bread is best as it makes the sandwich easier to eat.
Related Recipes
Did you try this Roast beef Sandwich?
Please leave me a ⭐ rating below and let me know in the comments how you enjoyed it.
Roast Beef Sandwich Recipe with Slow Roasted Tomatoes
Ingredients
- ½ small red onion - thinly sliced.
- ½ teaspoon butter - for cooking the onions. Butter for spreading on the sandwich is optional.
- ½ teaspoon olive oil
- 2 slices crusty sourdough or 1 small baguette - substitute bread of your choice
- 1 teaspoon wholegrain mustard
- 4 oz rare roast beef. - Thinly sliced. More or less, depending on your taste. (either leftover roast beef, pre-roasted or deli-bought slices)
- ⅓ cup watercress. - you can substitute with arugula or baby spinach.
- 5-6 slow-roasted tomatoes - cut in half and juice retained.
Instructions
- Place the thinly sliced onion in a frying pan with a small dob of butter and a drizzle of olive oil. Cook on medium heat, stirring often, being careful to sweat the onions, not burn them. Cook until the onions are soft and caramelized.Optional: As the onions soften, add a small drizzle of balsamic vinegar and continue to stir (the balsamic will help the onions become a nice, sweet, jam-like consistency).
- Spread the whole grain mustard on one slice of the bread, layer with slices of roast beef and top with the caramelised onion.
- Top with watercress and roasted tomato halves. Drizzle with any of the tomato juice before topping with the other slice of bread.
Notes
Recipe Variations
- Salad Greens: Baby spinach or arugula are good options for a roast beef sandwich.
- Mustard: You can use any mustard you prefer. Horseradish cream is also a good choice if you like some heat.
- Cheese: I don’t add cheese to this sandwich because I don’t feel it needs it. If you want to add cheese, cheddar, provolone, Swiss, and blue cheese are all good options.
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