• Heat cast iron skillet or pan over medium heat. Add in 1-2 tsp of oil and 1 tbsp of butter. Once the butter begins to bubble add in diced onions, salt and 1 tsp of sugar, optional and cook until browned and caramelized. Stirring frequently. Add water if needed. Once caramelized/grilled to your liking remove onions from the pan. Add in a little more oil if needed, then toast buns until golden, about 1 minute. Set aside.
• Bring ground beef to room temp and form beef into 4-6 patties on pieces of parchment paper. (form the patties slightly larger than your buns and not as thick as a normal patties). Season both sides with kosher salt and black pepper.
• Heat pan to medium-high heat. Add 1 tsp of oil and allow to heat up. Place patty or patties in the pan (depending on how large your pan is). Press down to get the full surface of the patty in contact with the skillet. Cook to your desired doneness without moving the meat, until browned and a crust forms. I cooked mine 2-3 minutes on both sides. On the exposed side of the patty, add 1 1/2 tsp of mustard, then flip the patty. Allow to cook for 2 another minutes.
• Add 1-2 slices of cheese. Add a little water to the pan and put the top on. Cook for about 1 minute (this helps the cheese melt)

For secret sauce

• Combine mayo, ketchup, mustard, sweet relish, vinegar, sugar and garlic paste. Mix until well combined. Taste and adjust to your liking.

To Assemble

• Spread the secret sauce on the bottom bun, then layer with tomatoes, lettuce, patty, grilled onions, raw onion, second patty (if you're making it a double-double), chili peppers, more sauce then the top bun.
• Serve with fries & enjoy!