Cook pasta according to package directions. Drain but do not rinse. Set aside
In a large saute pan over medium heat, melt 2 tablespoons of butter. Add the onions and saute for 2-3 minutes until they begin to get tender but not browned.
Add the mushrooms to the saute pan and cook until the mushrooms are lightly browned. Add the garlic and cook for another minute. Remove the mixture from the pan and set aside.
In the same pan, melt 4 tablespoons of butter over medium-high heat. Add the flour and cook for 1-2 minutes. Slowly, whisk in the cream, milk, and chicken stock. Continue cooking until the sauce is bubbling and thickened.
Stir in the parmesan cheese until well combined. Remove the pan from the heat.
Add the turkey, pasta, and frozen peas to the sauce. Mix well. Pour into a buttered 9x13 baking dish.
In a small saute pan, melt the remaining butter. Add the breadcrumbs and toss until they are thoroughly coated in the butter. Spoon evenly over the Turkey Tetrazzini.
Bake in a preheated 350-degree oven for 30-35 minutes until bubbly and browned.