Special equipment: a piece of cheesecloth; a rimmed baking sheet (a quarter baking sheet works well); a 3 1/2- to 4-inch round cutter
For the lobster stock with cooked lobster: Tie together the peppercorns, garlic, thyme and bay leaf in a piece of cheesecloth for a bouquet garnis.
Bring a large pot of water to a boil. Add the lobster and cook for 6 minutes. Shock in an ice bath to stop the cooking. Remove the lobster from its shell and reserve both the meat and the shell.
Smash the lobster shell with a mallet until crushed into smaller pieces. In a separate large pot, add the lobster and shrimp shells, if using. Pan roast the shells over medium-high until the shells become pink/red and are fragrant. Add the leek, onion and enough water to cover (about 2 quarts). Bring to a boil, skim any scum that rises to the surface, add the bouquet garnis and reduce to a simmer. Simmer for 30 minutes or up to 1 hour over low heat. Set aside 3/4 cup lobster stock and 1 cup lobster meat for use in the lobster salad. Refrigerate any leftover stock or lobster in sealed containers for another use. The stock can also be frozen.
Dice the lobster into small dice and set aside for use in the lobster salad.
For the lobster salad: Combine the celery, scallions, mayonnaise, yogurt, lemon zest, salt, cayenne, paprika and lobster in a medium bowl and toss gently to combine.
For the mac and cheese: Spray a 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 lobster stock and whisk to combine. Add the Cheddar, fontina, Gruyere, Parmesan and pasta and stir until coated. Pour out onto the prepared baking sheet and flatten to about 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 macaroni into eight 3 1/2- to 4-inch rounds. Beat together the eggs with the milk in a shallow bowl. Place 1 cup flour in a second shallow bowl and the panko in a third. Bread the macaroni 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 macaroni 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 the lettuce. Close the sandwich with another mac and cheese bun. Repeat with the remaining ingredients.