Preheat oven to 375°F.
Melt 2T butter over medium high heat in a pan.
Add onions and cook until translucent.
Butter (~1T) a small cookie sheet.
Put your meat between two parchment sheets and roll until thin. Cut into squares the size of your sliders.
Put your meat on the cookie sheet and cook in oven for 10 minutes or until done.
Remove the patties from the cookie sheet.
Butter a casserole dish. Place the bottoms of the sliders in the casserole dish. Place meat, cheese, pickles, onions, and mustard on the sliders and top with the top half of the sliders.
In order to create the squashed look, melt your remaining butter and butter the tops of your sliders. Cover with a sheet of parchment and place another casserole dish on top or something heavy that is oven safe to squash the burgers down.
Bake for 5 minutes. ENJOY!