Preheat oven to 350ºF. Prepare pasta according to package directions. While pasta is cooking, melt 1 tablespoon margarine in skillet over medium-high heat. Add mushrooms and sauté 2 to 3 minutes. Drain off any liquid and set mushrooms aside. Place the remaining margarine in the skillet and melt over medium-low heat. With a wire whisk, stir in flour. Gradually add the Dijon mustard, milk and salt, stirring constantly until mixture boils and thickens. Once thickened, stir in mushrooms and remove from heat. When pasta is done, drain well. Return pasta to the pot and add chicken, mushroom mixture, pimentos, cheese and pepper. Combine all ingredients together and transfer to a 2-quart casserole dish. Bake for 15 minutes or until heated through. Serve immediately. Serves 4 to 6