Preheat Oven: Preheat the oven to 350°F and grease a rimmed baking sheet with butter.
Cook Beef: Heat olive oil in a large skillet over medium-high heat. Add diced onion and cook until soft. Add ground beef, salt, pepper, and garlic powder. Cook, breaking up the beef with a spatula, until browned. Drain excess fat.
1/2 tablespoon olive oil, 1/2 large yellow onion, 2 pounds lean ground beef, 1 teaspoon salt, 1 teaspoon black pepper, 1 teaspoon garlic powder
Mix in Mayo: Remove beef from heat and stir in mayonnaise until well combined.
1/4 cup mayonnaise
Prepare Buns: Slice dinner rolls in half. Place the bottom halves onto the greased baking sheet.
24 dinner rolls
Layer Cheese and Beef: Arrange sliced cheddar over the bottom buns. Spread the cooked beef mixture evenly on top, then sprinkle with shredded cheddar.
8 slices medium cheddar cheese, 6 ounces medium cheddar cheese
Top with Buns: Place the top halves of the rolls over the beef.
Brush and Bake: Brush melted butter over the buns and sprinkle with sesame seeds. Bake uncovered for 12-15 minutes, until the cheese is melted and the tops are golden brown.
2 tablespoons unsalted butter, 1 tablespoon sesame seeds
Serve: Remove from the oven and let cool slightly before serving.