Preheat Oven & Prepare Casserole Dish: Set your oven to 400ºF (200ºC). Grease a 9x13 inch rectangle casserole dish with butter or oil cooking spray. Set aside.
Thaw Veggies Partially: Take out the frozen vegetables and set them aside at room temperature. No need to completely thaw, just allow them to become slightly less frozen.
Cook Pasta: Bring a pot of water (half-filled) to a boil, add 1/2 tablespoon of salt, and then add the pasta. Cook according to the packet instructions until al dente, which took me about 9 minutes. Once cooked, drain the pasta and set it aside.
Melt Butter: In a large deep skillet or pot, melt butter over low heat.
Make Roux: Once melted, whisk in the flour. Increase the heat to medium, and cook the mixture for 1 minute as it bubbles/simmers, whisking continuously.
Add Milk: Gradually pour in the milk, about 1/2 cup at a time, whisking well after each addition. Continue cooking and whisking over medium - high heat until the sauce thickens, taking care to not let it boil.
Season: Once thickened, reduce the heat to low, add ground nutmeg and season with salt and pepper to taste. Whisk well.
Add Cheese: Add 2 cups of cheese and whisk until melted through.
Combine, Veggies, Pasta & Cheese: Add the partially thawed vegetables and the cooked pasta to the white sauce. Mix until everything is well coated.
Assemble Casserole: Transfer the mixture into the greased casserole dish. Sprinkle the remaining 1 cup of cheese evenly over the top.
Bake: Bake for 35-40 minutes, or until the top is golden and the casserole is bubbling.
Serve: Remove from the oven and let it side for 5 minutes before serving. Enjoy!