Adjust an oven rack to the middle position and heat oven to 400 degrees.
In a large pot or Dutch oven, bring 4 quarts water and 1 tablespoon salt to boil. Add pasta and cook until al dente, about 7 to 9 minutes. Drain well and rinse with cold water to stop the cooking.
Return the same large pot to medium-high heat and melt butter until foaming. Add onions and cook until softened, about 5 minutes. Stir in garlic and thyme until fragrant, about 30 seconds.
Stir in flour and cook until golden, stirring constantly, about 1 minute. Whisk in broth and half and half. Bring to a simmer and continue to whisk until sauce thickens, about 1 minute longer.
Remove from heat and whisk in Parmesan cheese. Season to taste with salt, pepper, and hot sauce if desired. Do not skip this step or your casserole may be bland!
To the pot with sauce, add cooked pasta, turkey, and frozen peas, and toss to combine. Pour into a 2-quart baking dish and sprinkle with breadcrumb topping.
Bake until the sauce is bubbling and the topping has browned, about 10 to 15 minutes. Cool 5 minutes before serving.