Preheat to 400°. Arrange bacon on a foil-lined baking sheet. Bake until crispy, 25-30 minutes. Transfer to paper towels to drain.
Form the ground meat into eight patties (1/4 pound each). Cover with plastic wrap and refrigerate for 20 minutes.
In a medium skillet, toss the onions with olive oil and sauté over medium-low heat until softened, about 20 minutes.
Heat 2 inches of canola oil in a deep saucepan to 350°.
Prepare three separate bowls: one with the egg and buttermilk (whisked together), one with ¼ cup flour, and one with the remaining flour, cornmeal, salt, pepper, and cayenne (mixed together).
Working in batches, dredge tomato slices in the plain flour, then the egg wash, and then the cornmeal mixture until evenly coated. Fry tomatoes in batches until golden brown, about 2 minutes. Transfer to a paper towel-lined plate until ready to serve.
Light a grill or preheat a grill pan. Season beef patties generously with salt and pepper and brush with olive oil. Grill over moderately high heat until well-browned on the bottom, 5-6 minutes. Flip the burgers and grill for 5-6 minutes longer, or until cooked to desired doneness.
Top each bottom bun with some lettuce, followed by a burger patty, a spoonful of warm queso, a second burger patty, more queso, then the bacon, onions, and fried green tomatoes. Top with top bun and serve immediately.