Heat oven to 425°F and spray a large casserole dish with nonstick baking spray.
Cook the fettuccini in salted water, to al dente (per package directions), drain and rinse with cold water. Spread the noodles evenly onto a large baking sheet to dry; set aside.
1 pound dry fettuccini noodles
While the fettuccini cooks, heat a large skillet over medium heat and add 2 tablespoons oil and 2 tablespoons butter. When the butter has melted, add mushrooms and onion; season lightly with salt and pepper. Sauté until onions are translucent and mushrooms are fragrant and have softened, about 5-7 minutes. Stir occasionally.
2 tablespoons olive oil, 18 tablespoons unsalted butter, ½ pound button mushrooms, 1 medium yellow onion
Add garlic, thyme and white wine; stir with a wooden spoon while scraping off any browned bits on the bottom. Cook an additional 2 minutes to reduce the mixture. Transfer the vegetable/wine mixture to a medium bowl; set aside. (Keep the sauté pan out for later use.)
4 cloves garlic, 1 tablespoon chopped fresh thyme leaves, ½ cup semi-dry or dry white wine
In a medium saucepan, set over medium heat, bring the milk, cream and broth to just below a simmer (when you see steam coming off the top).
3 cups whole milk, 1 cup heavy cream, 1 cup low-sodium chicken broth
In the large sauté pan, melt 3 tablespoons butter over medium heat. While whisking, sprinkle the flour over the butter and continue whisking until the flour and butter incorporate and turn a light golden brown, about 2 minutes.
⅓ cup all-purpose flour
Continue whisking and slowly add the milk/cream/broth mixture. The mixture should be smooth with no lumps. Add nutmeg, seasoned salt, kosher salt and black pepper. While stirring, bring the mixture just to a boil and immediately reduce the heat to low. Cook, while whisking continuously, another 8-10 minutes, or until the sauce thickens. Remove the sauce from the heat once it thickens.
⅛ teaspoon freshly grated nutmeg, 1 teaspoon seasoned salt, 1 teaspoon kosher salt, ½ teaspoon freshly ground black pepper
Add the chicken, mushroom/onion mixture, noodles, and half of the parsley. Mix well.
3 cups cubed rotisserie or roast chicken meat, ⅓ cup chopped fresh parsley
Pour the mixture into the sprayed dish and sprinkle evenly with breadcrumbs and Parmesan.
½ cup grated Parmesan cheese, ½ cup breadcrumbs
Melt the remaining butter and pour it evenly over the top.
Bake at 425°F, for 30 minutes, or until the top is golden brown and the chicken tetrazzini is bubbly throughout. Remove from oven and let the tetrazzini rest 5-10 minutes before serving.
Garnish with the remaining parsley before serving.
Enjoy!