For the doctored fry sauce: Combine the ketchup, mayonnaise and hot sauce in a small bowl. Set aside.
For the quick and easy mac and three cheeses: Bring a pot of water to a boil. Add the macaroni and cook according to the package directions.
Heat the milk and butter in a large saucepan over medium heat until hot. Add the fontina, Cheddar jack and processed cheese and cook, stirring, until melted. Stir in the pepper and salt. Drain the macaroni and stir it into the cheese sauce. Keep warm.
For the onion strings: Begin by peeling the onions and slicing them very thinly; use a mandoline slicer if you can. Separate the onion slices, put them in a shallow baking dish and pour the buttermilk over the top. Press the onions down so they're submerged as much as possible and let them soak on the counter for at least an hour. If necessary, stir them around a bit halfway through the soaking process.
In a bowl, combine the flour, cayenne, kosher salt, seasoned salt and black pepper and set aside.
Pour 1 to 2 quarts canola oil in a pot or Dutch oven. Heat to 360 degrees F, or until a pinch of flour sizzles when sprinkled over the pot.
Using tongs, remove a handful of onions from the buttermilk and immediately dunk into the flour mixture. Coat the onions in the flour mixture, then tap them on the inside of the bowl to shake off the excess.
Plunge the onions in the hot oil. With a spoon, fiddle with them a bit just to break them up and watch them! They'll be ready to remove from the oil in no time flat. When they're golden brown (this should take just a minute or two), remove them from the oil and place on a paper towel-lined plate. Repeat with the remaining onion slices and serve immediately.
For the burger: To a bowl, add the ground beef, salt, pepper and hot sauce and mix to combine. Shape into two 10-ounce burger patties.
Melt the butter in a skillet or grill pan over medium-high heat and cook the burgers for 5 minutes per side for medium rare.
For the burger bun: Melt the butter in a skillet or grill pan over medium heat. Put the bun halves in the skillet cut-side down and lightly toast, about 2 minutes.
To build the burger, layer on top of the bottom bun: the butter lettuce leaves, some mac and cheese, 1 burger patty, the bacon, the American cheese slices, 1 tomato slice, the other burger patty, some onion strings, a spoon of the doctored fry sauce, spinach leaves, the other tomato slice, the pickles and the top bun. Secure with a large metal skewer.