Preheat your oven to 350°F (175°C).
Lightly butter the bottom of a rimmed baking sheet to prevent sticking.
Heat a large skillet over medium-high heat and add ½ tablespoon of olive oil.
Toss in the diced onion and 2 pounds of ground beef. Use a spatula to break up the beef into small pieces.
Season with 1 teaspoon each of salt, black pepper, and garlic powder.
Cook until the beef is just cooked through, then remove the skillet from the heat.
Tilt the skillet to spoon off and discard any excess fat.
Stir ¼ cup of mayonnaise into the cooked ground beef until well combined.
Slice the dinner rolls in half horizontally. Place the bottom halves on the prepared baking sheet.
Layer the bottom halves with sliced cheese, then spread the ground beef mixture evenly over the cheese. Use the back of a spatula to shape the beef into a neat, even layer.
Top the beef with 6 ounces of shredded cheddar cheese.
Place the top halves of the rolls over the beef and cheese, cut side down.
Brush the tops with 2 tablespoons of melted butter and sprinkle generously with sesame seeds.
Bake at 350°F for 12–15 minutes, or until the cheese is melted and the tops are golden brown.