Preheat the oven to 350°F (175°C). Lightly grease or line a baking dish or sheet pan that can hold the slider buns snugly.

Heat the olive oil in a large skillet over medium-high heat. Add the diced onion and bell pepper, then cook for 3 to 4 minutes until softened.

Add the ground beef and cook, breaking it up with a spatula, until browned and no longer pink, about 6 to 8 minutes.

Stir in the garlic, salt, black pepper, Worcestershire sauce, and ketchup. Cook for 1 minute more, then remove the skillet from the heat.

Slice the slider buns in half horizontally, keeping the top and bottom halves intact if possible. Place the bottom halves in the prepared baking dish.

Spread mayonnaise and mustard over the bottom buns if using. Spoon the chopped cheese mixture evenly over the buns, then layer the American cheese slices on top.

Place the top buns over the filling. Brush the tops with melted butter and sprinkle with sesame seeds if desired.

Bake for 8 to 10 minutes, until the cheese is melted and the tops are lightly toasted. Serve warm and cut into individual sliders.