Preheat oven to 350 degrees. Cut sliders rolls in half equally and horizontally. In a small bowl combine the ketchup, mayo, and mustard and set aside.

On a large piece of parchment paper, start by forming ground beef into two equal size balls. Place another piece of parchment paper on top of ground beef balls and smash flat using a large baking sheet or casserole dish until the patties are thin and large enough to fill the size of the slider rolls. Form the meat into a rectangle that will fit the size of the slider rolls. Make sure to account for shrinkage that will occur when cooking. Season meat generously with burger seasoning or charbroil seasoning.

Baste the interior of the sliders rolls with butter and place in the oven for 5-10 minutes until lightly toasted. In a large skillet or griddle over medium-high heat place the patties and allow to cook to your preference. Once finished cooking leave some grease in the pan and add the diced onions. Sautee onions for 5-10 minutes until soft and lightly golden brown. Remove from skillet and set aside.

Start layering the burger in this order: bottom portion of the slider rolls, hamburger patty, 4 slices of cheese, hamburger patty, 4 slices of cheese, burger sauce, sauteed onions, top portion of slider rolls. Baste the top of the sliders with melted butter and place back into the oven for 5-10 minutes or until cheese is melted and top is toasted. Serve & Enjoy!