Tender, thinly sliced roast beef on a crusty roll with savory au jus for dipping makes this high protein sandwich irresistible. The French dip is a family-friendly classic that's simple yet deeply satisfying.
Preheat the oven to 375°F. Slice the rolls in half and lightly butter the cut sides.
Finely slice the onion. In a medium saucepan over medium heat, melt 1 tbsp butter and sauté the onion until deeply caramelized, about 10 minutes, stirring occasionally for even browning.
Add the beef broth, Worcestershire sauce, soy sauce, garlic powder, black pepper, and salt. Bring to a simmer and let cook for 10 minutes to develop flavors. Strain onions if desired for a classic clear au jus.
Place sliced roast beef in the hot au jus for 2-3 minutes to warm gently without boiling—this keeps the beef tender.
Toast the buttered rolls cut-side up in the oven for 3-4 minutes until crisp. Pile warm beef onto rolls, top with caramelized onions and cheese if desired, close the sandwiches, and serve immediately with small bowls of hot au jus for dipping.