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.
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.
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.)
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).
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.
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.
Add the chicken, mushroom/onion mixture and half of the parsley. Mix well.
Pour the mixture into the sprayed dish and sprinkle evenly with breadcrumbs and Parmesan.
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.