Preheat your oven to 350°F (175°C).

Carefully slice your slider buns horizontally, being careful not to fully separate them.

In a skillet, sauté the sliced onions (and bell peppers if using) over medium heat for 8-10 minutes until caramelized.

In the same skillet, add the ground beef over medium-high heat and cook until browned.

Drain any excess fat from the skillet.

Return the sautéed onions (and bell peppers) to the skillet with the beef, and season with salt and black pepper.

Arrange the bottom halves of the slider buns in a baking dish.

Spoon the beef and onion mixture evenly over the bottom buns.

Layer provolone cheese slices over the beef mixture.

Place the top halves of the slider buns over the cheese.

Melt the unsalted butter and brush it over the top of the buns.

Bake for 15-20 minutes, or until the cheese is melted and the tops of the buns are golden brown.

Remove from the oven, slice into sliders, and serve warm.