Begin by cooking your pasta according to the package instructions. Undercook it by about two minutes since it will continue cooking in the oven. Drain and set aside.

In a large skillet, melt the unsalted butter over medium heat. Add the diced onion and minced garlic. Sauté for about 3-4 minutes, until the onion becomes translucent. Then, incorporate the sliced mushrooms and cook for another 5 minutes until they are tender, releasing their moisture.

Sprinkle the flour over the vegetables and mix to combine. Slowly pour in the chicken stock whilst stirring to prevent lumps. Allow this to simmer for several minutes until the mixture starts to thicken. Then stir in the heavy cream and sherry. Bring the mixture to a gentle simmer and cook for an additional 3-4 minutes.

Once the sauce thickens, add the diced turkey, peas, and chopped thyme. Stir until everything is well mixed and heated through. Season generously with salt and pepper to taste.

Now, add the cooked pasta to the turkey mixture. Stir gently until the pasta is fully coated in the sauce.

Preheat your oven to 350°F (175°C). Transfer the pasta mixture into a greased baking dish, spreading it evenly.

Sprinkle the grated parmesan cheese evenly across the top. Then, layer the breadcrumbs on top, creating a crunchy topping that will brown beautifully in the oven.

Bake in the preheated oven for about 25-30 minutes or until the top is golden brown and bubbly.

Take the baking dish out of the oven and allow the tetrazzini to cool for a few moments. Serve it warm and watch as it disappears!