Special equipment: a rimmed baking sheet (a quarter baking sheet works well) and a 3 1/2- to 4-inch round cutter
For the lobster salad: Combine the lobster, celery, scallions, mayonnaise, yogurt, lemon zest, salt, cayenne and paprika in a medium bowl and toss gently to combine.
For the sandwich: Spray a rimmed baking sheet lightly with cooking spray. (A quarter baking sheet works well.)
In a large 6 quart pot, bring water to a boil. Cook the elbow macaroni according to the package directions. Drain well.
Meanwhile, melt the butter in a large saucepan over medium heat. Add the onion, garlic and salt and sautee, stirring, until soft, 2 to 3 minutes. Add 2 tablespoons flour and cook, stirring constantly, until lightly browned, 2 minutes. Add 3/4 cup milk and whisk to combine. Add the Cheddar, fontina, Gruyere, Parmesan and pasta and stir until coated. Spread out the mixture on the prepared baking sheet and flatten to between 1/2 and 3/4 inch thick. (The mixture will not fill a standard baking sheet but it will fill a quarter baking sheet). Chill, covered with plastic wrap, for at least 2 hours or overnight.
Cut the mac and cheese into eight rounds using a 3 1/2- to 4-inch cutter. Beat together the eggs with the remaining 1/4 cup milk in a shallow bowl. Place 1 cup flour in a second shallow bowl and the panko in a third. Bread the mac and cheese buns, dipping first in the flour, then the egg wash and then the panko.
Heat a large nonstick skillet with 1 tablespoon olive oil over medium-high heat. Sear each mac and cheese bun until golden, 1 1/2 to 2 minutes per side. Set aside to use as bread for the sandwich.
Wrap each tomato slice in one piece of prosciutto. Preheat a saute pan over medium high heat. Add a small amount of olive oil (less than 1 tablespoon) and swirl to coat the pan. Add the prosciutto-wrapped tomatoes and sear until browned, about 30 seconds per side. Remove to a plate.
To build the sandwich: Place one browned mac and cheese bun on a work surface. Add a prosciutto-wrapped tomato, 1/3 cup lobster salad and top with some lettuce. Close the sandwich with another mac and cheese bun. Repeat with the remaining ingredients.