Preheat the oven to 200 degrees C. Bring a large pot of lightly salted water to a boil. Add the pasta and cook for 4 minutes, or until almost tender. Drain.
Melt the butter in a saucepan over medium heat. Add the onion; cook and stir until tender. Stir in the flour until blended, then gradually stir in the milk so that no lumps form. Season with salt, pepper, chicken seasoning and mustard. Cook over medium heat, stirring constantly until the mixture thickens. Remove from the heat and add 2/3 cup cheese, stirring until cheese melts. Add mushrooms.
Place a layer of pasta in the bottom of a 20x30cm baking dish. Cover with a layer of turkey, and then a layer of cheese sauce. Repeat the layers. Sprinkle remaining 1/3 cup cheese over top.
Bake for about 25 minutes in the preheated oven, until sauce is bubbly and cheese on top is toasted.