1. In a skillet, heat vegetable oil on high and caramelize the onions. Season with salt and pepper; cook until dark in color. Remove cooked onions when finished, set aside. DON’T clean skillet.
2. Prepare the sauce. In a medium bowl, combine all sauce ingredients, mix well, and set aside.
3. Using the same skillet that you cooked the onions in, over high heat add a little oil and place flattened meat patties in skillet. Season with salt and pepper while in the pan.
4. Cook on each side for 1 minute, flipping twice. While finishing the cook time on the second flip, add 1 slice of American cheese on each patty. Cut heat on skillet and let meat rest in pan until cheese is melted.
5. Assemble burger in this order: bottom bun, burger sauce, lettuce, caramelized onions, tomato, stacked cheesy patties, top bun.