Heat a large skillet over medium heat. Add 2 tablespoons butter. Once butter is melted, add sliced onions. Cook, stirring occasionally for 15 to 20 minutes until the onions are softened and turning a golden brown color. (You don't want to sear the onions, but cook them slowly.)
While the onions cook, Heat your grill or a grill pan over medium-heat heat.
To a medium mixing bowl add ground beef, salt, pepper, onion powder, and paprika. Use your hands to mix the beef and spices until evenly combined. Divide the beef in half and form each portion into a patty.
Cook burger patties for 4 to 5 minutes per side, turning once, until medium in the center. (Or cook to your preferred doneness.) Remove burgers from heat and top each burger with half of the blue cheese.
While the onions finish cooking, spread 1/2 tablespoon butter on each cut side of the burger buns. Place on a baking sheet, buttered side up, and broil on HIGH for 1-3 minutes or until golden and toasted. Removed from the oven. (You could also toast your buns on the grill, or leave them untoasted.)
Assemble the burger:
Place half the lettuce on the cut side of the bottom burger bun. Place a patty on the lettuce. Top the blue cheese with half of the caramelized onions. Drizzle 1 tablespoon of hot sauce over the onions. Coat the cut side of the top bun with half the mayo and place on top of the burger. Serve immediately.