Preheat oven to 350 degrees F. Line a cookie sheet with foil. Cut slider buns in half. Place 12 halves on cookie sheet, and set the other halves aside.
In a large skillet, brown ground beef and onions. Cook until no longer pink. Stir in hamburger seasoning.
Spoon beef mixture evenly onto prepared buns. Next, add bacon, cheese pickles, ketchup, and mustard. Finally, top off with remaining buns.
Cover with foil and bake for 15 minutes. Remove foil, brush buns with butter and bake for 5-10 more minutes.