Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.
Step 2: Cook spaghetti or linguine according to package directions until al dente. Drain well, toss with a drizzle of olive oil to prevent sticking, and set aside.
Step 3: In a large pot or Dutch oven, melt the butter over medium heat. Add the diced yellow onion and cook until softened and translucent, about 5-7 minutes.
Step 4: Add the sliced mushrooms to the pot. Cook, stirring occasionally, until the mushrooms have released their moisture and are lightly browned, about 8-10 minutes.
Step 5: Sprinkle the flour over the cooked onions and mushrooms. Cook, stirring constantly, for 1-2 minutes to create a roux. This will help thicken the sauce.
Step 6: Gradually whisk in the chicken broth, ensuring no lumps form. Bring the mixture to a gentle simmer, stirring continuously, until it begins to thicken.
Step 7: Stir in the heavy cream, cream of mushroom soup (if using), garlic powder, onion powder, black pepper, and salt to taste. Reduce heat to low and simmer for 5 minutes, allowing the flavors to meld. Taste and adjust seasonings as needed.
Step 8: Remove the sauce from the heat. Stir in the shredded cooked chicken and 1/2 cup of the grated Parmesan cheese.
Step 9: Add the cooked spaghetti to the sauce mixture. Toss gently until the pasta and chicken are evenly coated with the creamy sauce.
Step 10: Transfer the entire mixture to the prepared 9x13 inch baking dish. Sprinkle the remaining 1/2 cup of Parmesan cheese over the top, followed by the breadcrumbs.
Step 11: Bake for 25-30 minutes, or until the casserole is bubbly and the topping is golden brown. If the topping browns too quickly, you can loosely cover it with aluminum foil.
Step 12: Let the chicken tetrazzini rest for 5-10 minutes before serving. Garnish with fresh chopped parsley if desired, and enjoy this comforting meal!