These easy-to-make sandwiches are juicy, cheesy, and loaded up with amazing flavors from leftover roast beef! Whether you're making one for lunch, trying to use up leftovers, or you're making a dozen for game day, Roast Beef and Cheese Sandwiches are a guaranteed winner!
In a medium saucepan, add beef and stock. Bring to a low simmer. Continue to cook for approximately 10 minutes, or until most of the liquid has reduced.
Melt 1 tablespoon of butter in a saucepan, and add onion. Season with salt and pepper. Cook onion until just slightly brown.
Heat oven to 375 degrees. Gently open hoagie rolls, and spread butter on the insides of each. Place on a sheet tray and toast until golden brown. Remove from oven.
Increase oven temperature to 425 degrees.
Using tongs or a slotted spoon, place a large helping of roast beef on each roll. Sprinkle with cheese. Place in oven for 5–8 minutes, or until cheese is melted.
Remove from the over, top with onions, and serve immediately.