Heat oven to 400 degrees.
In a bowl, combine beef, onion, bbq sauce, bread crumbs, seasoned salt and pepper. Gently press mixture into a 9 x 13 inch baking dish so that it makes one large patty of even thickness. Use the tines of a fork to poke holes throughout the meat.
Bake for 30 minutes.
When the hamburger is fully cooked, the meat will shrink away from the sides and liquid will accumulate around the edges of the pan.
Remove the pan from the oven and carefully drain of the liquid from the pan.
Place slices of cheese in a single layer on top of the meat and return pan to oven for about 2-3 minutes or until the cheese starts to melt.
Remove from the oven and allow to rest for a few minutes. cut the meat into 2 inch squares (or the size of the buns used), making 12 mini hamburgers.
Serve with dill pickles, ketchup, mustard, tomatoes and lettuce.