How to Make It
Preheat oven to 400°F. Prepare pasta according to package directions, adding 1 tablespoon of the kosher salt to boiling water. While pasta cooks, melt butter in a large saucepan over medium. Whisk in flour; cook, whisking constantly for 1 minute. Gradually whisk in milk; cook, stirring constantly, until mixture thickens, about 10 minutes. Remove from heat, and whisk in pepper and remaining 1 teaspoon salt.
Add milk-flour mixture, Cheddar, 1 cup of the mozzarella, and 1/4 cup of the Parmesan to cooked pasta, and stir until well combined. Stir in eggs and panko. Evenly spread macaroni mixture in a 16-inch pizza pan lightly coated in cooking spray. Sprinkle remaining 1/2 cup Parmesan over macaroni mixture. Bake in preheated oven until lightly golden around edges and on top, 20 to 25 minutes.
Spread pizza sauce evenly over the “crust”, leaving a 1-inch border. Top with mushrooms, pepperoni, and remaining 1 cup mozzarella. Return to oven, and bake until cheese is melted and slightly bubbly, about 5 minutes. Sprinkle with basil and crushed red pepper, if desired.