Preheat oven to 425°F. Lightly coat a 9 x 13-inch casserole dish with cooking spray.
Cook the noodles in salted water according to the package directions. Drain well and set aside.
Melt 3 tablespoons of butter in a small bowl in the microwave in 20-second intervals. Add the breadcrumbs and stir until well coated.
In a large pot over medium-high heat, melt the remaining 3 tablespoons of butter. Add the onion, mushrooms, and garlic and cook until softened, about 6 minutes. Add the flour and cook, stirring constantly, for 1 minute. Gradually stir in the milk. Add the basil, salt, and pepper. Bring to a boil and cook, stirring frequently, until thickened, about 5 minutes. Remove from the heat and stir in 1½ cups of cheese until melted. Add the noodles, turkey, and peas. Stir until well combined.
Transfer the mixture to the prepared casserole dish. Sprinkle with the remaining ½ cup of cheese and top with the buttered breadcrumbs.
Bake for 15 to 20 minutes or until the topping is golden and the filing is bubbly. Garnish with chopped parsley before serving, if desired.
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Notes
Use a large pot. This makes it easy to mix everything together before transferring to the casserole dish.
Swap the butter for olive oil. For cooking the onions, mushrooms, and garlic, you can use olive oil instead of unsalted butter. I recommend sticking with butter to mix with the breadcrumbs, though.
Shred the cheese yourself. Pre-shredded cheese contains anti-clumping agents, so it doesn’t melt as nicely as freshly grated cheese. Plus, the blocks are typically less expensive per ounce than the pre-shredded bags!
You can use a different pasta. Instead of egg noodles, you can use short pasta like penne, macaroni, gemelli, rotini, or fusilli.
Don’t be concerned if the mixture looks very saucy. During baking, the noodles will absorb some of the liquid, and you’ll end up with a thick, creamy casserole.
Make a turkey casserole with rice. Cook 2 cups of long-grain rice, brown rice, or wild rice to use instead of noodles. Cook the rice until just tender instead of completely soft because it may end up mushy when baked if it was cooked for too long.
Add fried sage! For an extra dose of fall flavor, fry 12 sage leaves 2 tablespoons of oil in a small skillet. Reserve a few for garnish, then crumble the others and mix them into the pot along with the turkey.