Preheat oven to 350 degrees F (175 degrees C). Grease a 9x13 inch baking dish.
Melt butter in a saucepan over medium heat. Stir in flour; cook and stir until bubbly, about 2 minutes. Gradually stir in chicken broth, half-and-half, garlic powder, nutmeg, paprika, oregano, white pepper, and salt. Simmer until sauce thickens, about 5 minutes.
Stir chicken or turkey, mushrooms, Parmesan cheese, green bell pepper, peas, onion, olives, and almonds into the sauce. Place spaghetti in the prepared baking dish. Pour sauce over the spaghetti. Bake in the preheated oven for 30 minutes, or until bubbly and lightly browned.
Interesting Facts
Tetrazzini can be made with any type of cooked poultry or seafood.
Tetrazzini is an Italian-American dish named after the Italian opera singer Luisa Tetrazzini.
Tetrazzini can be served as a main course or as a side dish.