Preheat oven to 350˚F. Butter the bottom of a rimmed baking sheet.
Place a large skillet over medium/high heat with 2 tsp olive oil. Add diced onion then 1 lbs ground beef and break up with a spatula. Season with 1/2 tsp salt, 1/2 tsp black pepper, and 1/2 tsp garlic powder. Sautee over medium/high heat, breaking up the beef and cook just until cooked through then remove from heat.
Tilt skillet to spoon off and discard excess fat. Stir in 1/4cup mayo.
Place bottom half of dinner rolls on buttered baking sheet and line bread with sliced cheese. Spread ground beef mixture evenly over the sliced cheese, using the back of a spatula to square off the edges. Cover the ground beef with sliced cheese.