In a pan/skillet heat a little bit of oil, add in your diced onions and allow them to cook for about 3 to 4 minutes. Add in your beef break up and brown. Once cooked drain and set aside.
Mix all the sauce ingredients together in a medium bowl.
Preheat your oven to 350˚. Cut slider buns in half and lay bottom half on a baking sheet.
Spread the beef and onion across the bottom buns, next add your cheese slices, the sauce, ketchup, mustard and last pickle.
Lay the top of the buns over everything to close it up. Cover with foil and bake for about 15 minutes.
Remove sliders from oven and brush with melted butter, then sprinkle sesame seeds.
Place back into the oven for about another 5 minutes and they are ready to serve.