Heat a heavy-bottomed pan or skillet over medium heat. Add the ground beef and diced onions. Cook until beef is no longer pink, crumbling the meat as it cooks. Drain the excess fat.
To the browned meat, add the ketchup, mustard, Worcestershire sauce, salt, and pepper. Stir to combine. Set aside.
In a 9x13 casserole dish, arrange the bottom of the slider buns. Top with a layer of American cheese. Spread the beef mixture evenly over the buns and cheese. Top with the Pepperjack cheese. Arrange the pickle slices evenly over the sliders. Arrange the top of the slider buns on the top of the casserole.
Brush the buns with melted butter and sprinkle with dried minced onion or sesame seeds. Cover the casserole dish with aluminum foil. Bake for 30 minutes until cheese is melted and sliders are warm throughout. Serve.