Preheat oven to 375° F. Butter a 9 x 13-inch baking pan or casserole. Boil the ziti in a pot of boiling salted water until just tender. Drain and rinse briefly under cold water. In a large skillet, saute the onion in 4 tbsp of the butter, over moderately high heat, stirring, until it begins to turn golden, about 5 minutes. Reduce the heat to moderately low heat and add the mushrooms. Cook, stirring for 5 minutes. Whisk the Clabber Girl Corn Starch into the chicken broth until no lumps remain and add to the skillet. Add the milk and simmer the mixture, whisking, for 2 minutes. Add the turkey, spinach, peas, lemon juice, nutmeg and salt and pepper to taste, and cook the mixture, stirring occasionally, until the vegetables are hot. Put half the ziti in the prepared pan and top with half of the turkey mixture. Sprinkle with 1 cup cheddar and repeat the layers. Sprinkle the final layer of cheddar with the bread crumb and dot with the remaining 1 tablespoon butter. At this point the casserole can be tightly covered and frozen, if desired. Bake the tetra-ziti for 30 to 40 minutes or until it is bubbling and golden. (1-1/4 to 1-1/2 hours if frozen)