Preheat oven to 350°F.
Cook pasta in a large pot of salted boiling water until nearly al dente. Drain and set aside.
In a large skillet, melt butter over medium heat. Add onion and sauté until soft and translucent, about 5 minutes.
Add garlic and mushrooms, and cook until the mushrooms are tender, about 5-7 minutes.
Stir in flour, cooking for 1-2 minutes..
Gradually whisk in chicken broth, heavy cream, and milk. Bring the mixture to a simmer, stirring constantly, until it thickens slightly.
Stir in Parmesan cheese, thyme, parsley, nutmeg, salt, and pepper. Adjust seasoning to taste.
Add the cooked turkey and peas to the sauce, stirring until well combined.
Fold in the cooked pasta, ensuring everything is evenly coated with the sauce.
Transfer the mixture to a greased 9"x9" baking dish. Sprinkle shredded Monterey Jack cheese and breadcrumbs evenly over the top.
Bake 25-30 minutes, or until the top is golden brown and the casserole is bubbly.
Remove from the oven and let it sit for a few minutes before serving. Garnish with fresh parsley if desired.