1. Cook noodles in a lare amount of boiling salted water. Drain and transfer to a greased 13x9-inch baking dish, cutting long noodles to make serving easier.
2. In a saucepan, melt butter over medium-high heat. Add onion, mushrooms and celery and cook for 4 minutes, until softened. Stir in flour and cook for 2 minutes.
3. Add stock, evaporated milk, poultry seasoning, nutmeg, salt and pepper.
4. Cook stirring often, for 5 minutes, or until thickened. (sauce will still be quite thin). Stir in turkey and peas. Spoon mixture over noodles.
5. Top with cheese breadcrumbs and parsley. Bake in a preheated 350F oven for 30 minutes or until bubbling and browned. Let stand 10 minutes before serving.
6. TIPS: Tetrazzini can be assembled, covered and refrigerated several hours before baking.