In a medium saucepan over medium-high heat, cook bacon for 8-10 minutes until crispy. Use a slotted spoon to transfer bacon to a paper-towel-lined plate and set aside. Reduce heat to medium and add butter to the pan. Whisk in the flour gradually and cook, whisking constantly, for 1-2 minutes until the mixture turns a light golden color. Slowly incorporate milk, whisking continuously to prevent lumps, and bring to a simmer. Add the cheeses and whisk until fully melted. Stir in mustard, salt, pepper, and nutmeg. Mix in the cooked pasta, bay leaf, and bacon. Let the mixture cool and remove the bay leaf before serving.
Crispy Onions
Heat 2-3 inches of canola oil in a deep cast-iron skillet to 350°F. Line a plate with paper towels. Soak onion slices in buttermilk in a medium bowl for 10-12 minutes. In a large, shallow pan, mix flour, cayenne, paprika, garlic powder, 2 teaspoons salt, and pepper. Remove onions from buttermilk, shake off excess, and coat in the flour mixture. Fry in batches for 2 minutes or until golden brown. Transfer to a paper-towel-lined plate and season with remaining salt. Repeat with the rest of the onions.
Donkey Sauce
Mix mayonnaise, garlic puree, yellow mustard, kosher salt, black pepper, and Worcestershire sauce in a small bowl to make about ½ cup.
Burgers
Preheat a large cast-iron griddle or skillet over high heat. Season burger patties with salt. Place on the griddle, spaced 3-4 inches apart, and cook for 90 seconds without moving. Flatten each patty to ¼ inch thick with a heavy spatula. Cook for 3-4 minutes or until juices appear on the surface. Flip and cook for 2-3 minutes more. Top each burger with a generous 2 tablespoons of mac 'n' cheese and 2 slices of cheddar. Cover with a dome or an inverted metal bowl, add 2 tablespoons of water under the dome, and steam for 60 seconds to melt the cheese. Remove the dome and let the burgers rest on a platter.
In a small bowl, mix melted butter, garlic puree, salt, and pepper. Brush the cut sides of the buns with garlic butter and toast on the griddle for 45 seconds or until golden and crisp. Spread Donkey Sauce on the buns. Layer the bottom buns with pickles, red onions, patties, bacon, tomato, lettuce, crispy onions, and top buns.