In a small bowl, combine A1 sauce, Duke's mayonnaise, and garlic powder. Mix until smooth and creamy, then cover with plastic wrap and refrigerate for at least 10 minutes.

Heat olive oil in a skillet over medium heat. Add sliced sweet onions and mushrooms, sprinkle garlic powder, and sauté for about 4-6 minutes until caramelized.

Divide ground beef into four equal portions, shaping each into a 1/4 pound patty. Season both sides with seasoning salt. Preheat griddle on high heat, then place patties and smash edges.

Cook patties for 3-4 minutes on each side until browned and cooked to desired doneness. Place a slice of white cheddar cheese on each patty.

Toast the potato buns cut-side down on the griddle for about 1-2 minutes until golden.

Assemble burgers starting with the bottom half of the toasted bun, then add the cheesy patty, sautéed onions, mushrooms, and a drizzle of A1 sauce.