Preheat your oven to 375°F (190°C). Grease a 9x13 inch baking dish with butter or cooking spray.

Cook the spaghetti or linguine according to package directions until al dente. Drain well and set aside. (If your chicken isn't already cooked, boil or poach chicken breasts until cooked through, then shred or dice them. This can be done simultaneously with pasta if using two pots).

In a large skillet or Dutch oven, melt 2 tablespoons of butter over medium heat. Add the chopped onion and cook until softened, about 3-5 minutes.

Add the sliced mushrooms and cook until they release their liquid and start to brown, about 5-7 minutes. Add the minced garlic and cook for another minute until fragrant.

Sprinkle the flour over the onion and mushroom mixture, stirring constantly for 1-2 minutes to cook out the raw flour taste, creating a roux.

Gradually whisk in the chicken broth, then the heavy cream and milk. Bring the sauce to a gentle simmer, whisking constantly until it thickens, about 3-5 minutes.

Stir in 1/2 cup of grated Parmesan cheese, salt, pepper, and nutmeg. Cook for another minute until the cheese is melted and the sauce is smooth and creamy. Taste and adjust seasonings if needed.

Add the cooked chicken, drained spaghetti, and thawed frozen peas to the cream sauce in the skillet. Toss gently until everything is evenly coated.

Pour the mixture into the prepared 9x13 inch baking dish, spreading it evenly.

In a small bowl, combine the Panko breadcrumbs, 1/4 cup grated Parmesan cheese, and melted butter. Sprinkle this topping evenly over the chicken tetrazzini.

Bake for 25-30 minutes, or until the sauce is bubbly around the edges and the topping is golden brown. If the topping is browning too quickly, you can loosely tent it with foil.

Remove from oven and let stand for 5-10 minutes before serving. Garnish with fresh chopped parsley, if desired.