Step 1: Prepare the Pasta and Preheat the Oven
Preheat your oven to 375°F (190°C). Grease a large 9×13 inch (or equivalent 3-quart) baking dish. Bring a large pot of heavily salted water to a rolling boil. Add the spaghetti, breaking it in half first if desired. Cook according to package directions until it is al dente. It’s very important not to overcook it, as it will cook further in the oven. Once cooked, drain the pasta well and set it aside.

Step 2: Sauté the Mushrooms and Aromatics
In a large Dutch oven or a heavy-bottomed pot over medium-high heat, melt 2 tablespoons of the butter. Add the sliced mushrooms and cook, stirring occasionally, for about 8-10 minutes. Don’t overcrowd the pan; cook in batches if necessary. The mushrooms will first release their water, and then they will begin to brown and caramelize. This browning process is key for developing deep flavor. Once browned, remove the mushrooms from the pot with a slotted spoon and set them aside.

In the same pot, melt another 2 tablespoons of butter over medium heat. Add the chopped onion and cook for 5-7 minutes, until it becomes soft and translucent. Add the minced garlic and cook for another minute until fragrant. Be careful not to burn the garlic.

Step 3: Create the Rich and Creamy Sauce
Sprinkle the all-purpose flour over the cooked onions and garlic. Stir constantly for 1-2 minutes to cook out the raw flour taste. This forms the roux, which will thicken your sauce.

Slowly, and a little at a time, pour in the dry sherry (if using), whisking constantly to deglaze the pot and scrape up any browned bits from the bottom. Let it bubble for about 30 seconds. Gradually whisk in the chicken broth, followed by the warmed milk. Continue to whisk constantly to prevent any lumps from forming.

Bring the sauce to a gentle simmer, then reduce the heat to low. Let it cook for 5-8 minutes, stirring frequently, until it has thickened enough to coat the back of a spoon. Remove the sauce from the heat. Stir in the salt, black pepper, nutmeg, and 1/2 cup of the grated Parmesan cheese. Stir until the cheese is completely melted and the sauce is smooth. Taste the sauce and adjust seasoning if necessary. It should be well-seasoned.

Step 4: Combine All the Components
To the pot with the cream sauce, add the cooked and drained spaghetti, the cooked mushrooms, the shredded turkey, and the thawed peas. Gently fold everything together until the pasta and all ingredients are evenly coated in the luxurious sauce.

Step 5: Assemble and Bake the Casserole
Pour the tetrazzini mixture into your prepared 9×13 inch baking dish and spread it into an even layer. Sprinkle the remaining 1/2 cup of Parmesan cheese and the 1 cup of shredded Gruyère (or other melting cheese) evenly over the top.

In a small bowl, melt the final 2 tablespoons of butter. Add the panko breadcrumbs and toss to coat them evenly in the butter. Sprinkle the buttered breadcrumbs over the cheese layer.

Bake, uncovered, for 25-30 minutes, or until the sauce is bubbly around the edges and the breadcrumb topping is golden brown and crispy.

Step 6: Rest and Serve
Remove the casserole from the oven and let it rest for 10-15 minutes before serving. This allows the sauce to set up slightly, making it easier to serve. Garnish with fresh chopped parsley just before bringing it to the table.