In a large skillet over medium high heat, melt 1 tbsp of the butter and then add chopped onions to the pan. Season with salt and pepper, to taste. Cook until onions are translucent and have slightly browned. Remove from pan.
Divide the ground beef evenly into 4 balls, 4 oz each. In the same skillet over medium heat, add each burger to the pan and smash using a piece of parchment paper over the burger, and under a large spatula. Season the patties with garlic powder, onion powder, salt and pepper. Cook for 2 minutes per side, or until the burgers are cooked through and the edges are crispy. Top with cheese and allow the cheese to melt before stacking two burger patties on top of each other.
While the burgers are cooking, make your sauce by combining mayonnaise, ketchup, mustard, relish, garlic powder, salt, and pepper in small bowl. I also like to add chili powder for some heat!
Once the patties are done cooking, remove from skillet. Melt the remaining butter on medium high heat and then add brioche buns to the hot pan to toast, about 20-30 seconds.
Begin assembling your smash burgers by adding burger sauce to the bottom toasted bun and then stack with lettuce, tomato, grilled onions, followed by the cooked patties. Top with the other half of the toasted bun, and enjoy!
Tip: While burger is still warm, wrap it in aluminum foil for 1-2 minutes for more authentic flavor!

Smash Burgers with Creamy Burger Sauce