In a small bowl, combine dressing ingredients. Cover bowl and place in refrigerator until ready to build burgers.
In a frying pan over medium-high heat, add butter and garlic powder. Once melted add sliced onions. Cook for 7-8 minutes, stirring occasionally, until a majority of the onions are crispy.
In a mixing bowl, combine patty ingredients. Use a 2-inch scoop to make 12 meat balls.
In a large saute pan over medium high heat, add a meatball. Cover with a square of parchment paper and use a flat spatula to press the meatball into a patty (roughly ¼-inch thick). Cook with 2 minutes on each side and then remove from pan. Repeat with remaining meatballs.
Add three patties back to the pan, and top each with a slice of muenster. Add another patty on top of each and another slice of muenster. Cover pan with a lid and let cheese melt. Remove from pan to cutting board and repeat with remaining patties.
Build the Burger: Spread top and bottom bun with dressing. Start with bottom bun and layer with arugula. Add double patties and top with portion of fried onions. Then top bun and press down slightly to hold together.