You will need:
- 2 microwave-safe 8-inch square baking dishes for this recipe
- 4 slices white sandwich bread, torn into pieces
- 1/4 cup grated Parmesan cheese
- 1 garlic clove, minced
- 8 Tbsp. (1 stick) unsalted butter, melted
- Salt and Pepper
- 1 lb. elbow macaroni
- 6 Tbsp. all-purpose flour
- 1 tsp. dry mustard
- 1/8 tsp. cayenne pepper (optional)
- 4-1/2 cups low-sodium chicken broth
- 1-1/2 cups heavy cream
- 4 cups shredded colby cheese
- 2 cups shredded extra-sharp cheddar cheese
Cook pasta: Bring 4 quarts water to boil in large pot over high heat. Add 1 Tbsp. salt and macaroni and cook until barely softened, about 3 minutes (important to avoid mushy blown-out macaroni). Drain pasta, then spread out on rimmed baking sheet and let cool.
Make sauce: Heat remaining butter, flour, mustard and cayenne (if using) in empty pot over medium-high heat, stirring constantly, until golden and fragrant; about 1 to 2 minutes. Slowly whisk in broth and cream and bring to a boil. Reduce heat to medium and simmer until slightly thickened, about 15 minutes. Off heat, whisk in colby, cheddar, 1 tsp. salt, and 1/2 tsp. pepper until smooth.
Freeze: Stir cooled pasta into sauce, breaking up any clumps until well combined. Divide pasta mixture between two 8-inch square baking dishes. Cool to room temperature. Wrap dishes tightly with plastic, cover with foil, and freeze for up to 2 months.
Bake: Preheat oven to 375 degrees F. Remove foil from casserole and reserve. Microwave casserole until mixture is thawed and beginning to bubble around edges, 7 to 12 minutes, stirring and replacing plastic halfway through cooking. Discard plastic and cover pan with reserved foil. Bake 20 minutes, then remove foil and sprinkle with 1 bag frozen bread crumbs. Continue to bake until crumbs are golden brown and crisp, about 20 minutes longer. Let cool 10 minutes.