Combine the onions and buttermilk, and refrigerate 8-12 hours.
Heat oven to 250°F.
Heat a few inches of oil to 350°F or set up a tabletop fryer. Combine flour and cornmeal and toss in O-rings to coat. Add to the hot oil and fry 3 minutes to golden. Season with salt and drain on paper towels. Keep warm in oven on a wire rack set onto a baking sheet.
Combine meat and spices, grated onion, garlic, Worcestershire, mustard, salt and pepper. Form 4 patties thinner at center and thicker at edges for even cooking.
Heat a cast-iron skillet over medium-high heat. Combine ketchup and Sriracha sauce in a small bowl.
Brush pretzel buns with a little water and sprinkle with the salt included in the package. Warm pretzel buns in the oven for 5 minutes and split them open.
Cook burgers in the hot skillet for 8 minutes, turning once.
Serve burgers on a bed of shredded lettuce topped with 2-3 crispy rings and some spicy ketchup.