Preheat oven to broil.
Slice the Hawaiian rolls horizontally and place them cut-side up on a baking sheet.
Melt 4 tablespoons butter and brush onto the inside of the rolls. Broil for 3–4 minutes until golden brown (watch closely).
Reduce oven temperature to 350°F (175°C).
In a large bowl, mix ground beef with salt and pepper.
Press the beef evenly into a 9×13-inch baking dish. Bake for 20 minutes.
While the beef cooks, sauté diced onions in 1 tablespoon butter over low heat until caramelized, about 15 minutes.
Remove cooked beef and drain excess juices.
Assemble sliders: place pickles over the bottom buns, then carefully add the cooked beef layer.
Top with a double layer of cheese slices, followed by sautéed onions.
Place the top buns over the sliders.
Brush tops with remaining 1 tablespoon melted butter and sprinkle with sesame seeds if desired.
Bake for 15 minutes, or until cheese is melted and sliders are heated through.