Heat 2 inches of oil over medium-high heat in a medium heavy-bottomed saucepan to 365 degrees F.
While the oil is heating, add the bacon to a large skillet over medium-high heat and cook until crispy and brown on both sides, about 10 minutes. Remove to a paper towel-lined plate. Reserve 2 tablespoons bacon fat in a small bowl and set aside to cool.
Place the onions in the buttermilk. Add the flour to a bowl or shallow baking dish. Scoop out a few onions at a time and dredge in the flour, shaking to remove any excess flour. Add to the hot oil and fry until golden brown and crispy, about 4 minutes, then remove to a baking sheet with a wire rack and sprinkle with House Seasoning while still hot. Repeat with the remaining onions and set aside.
Using your hands, mix the ground beef along with the cooled, reserved bacon fat and the garlic powder. Form the mixture into 4 loosely packed equal-sized patties with a slight dimple in the center to prevent them from doming. Place onto a baking sheet or large plate and sprinkle both sides of each patty generously with salt and pepper. Heat the skillet that was used to cook the bacon over medium heat. Add the burger patties, dimple side up, and cook on the first side, 5 to 6 minutes. Flip the patties and cook until the internal temperature of the burger is 135 to 140 degrees F for medium (the internal temperature will continue to rise when you remove the burger from heat to reach the ideal temperature of 145 degrees F), about 5 minutes more. Remove the patties from the skillet.
Add the brioche buns to the same skillet and toast until lightly browned.
To build the burgers, add one patty on top of a toasted brioche bun half. Drizzle over a couple tablespoons of the Blue Cheese Dressing. Top with a few fried onions, a slice of iceberg lettuce, another drizzle of Blue Cheese Dressing and two slices bacon. Top with the second half of the bun and place a cherry tomato on top, using a steak knife as a skewer to hold the burger together. Serve immediately.
Yield: 1 1/4 cups
Stir together the garlic powder, onion powder, paprika, salt and pepper in a medium bowl. Keep in an airtight container for up to 6 months.
Blue Cheese Dressing:
Yield: 1 1/2 cups
Whisk together the buttermilk, sour cream, minced chives, garlic salt and black pepper in a small bowl until well combined. Stir in the blue cheese crumbles. Refrigerate until ready to use.