Preheat oven to 425F. Lightly oil a 3-quart baking dish or coat it with nonstick spray.
Cook fettucine in boiling salted water until al dente, about 8 minutes. Drain and refresh under cold running water. Drain and set aside.
Meanwhile, in a large skillet, heat oil over medium heat. Add leeks and cook, stirring until lightly browned, 6 to 8 minutes. Add mushrooms and 1/2 teaspoon rosemary; cook stirring, until softened, about 5 minutes. Add flour and cook, stirring, for 1 minute. Add chicken broth and milk; bring to a boil, stirring, until slightly thickened, about 4 minutes more. Remove form heat and stir in chicken. Season with lemon juice, salt and pepper.Gently mix reserved fettucine into chicken mixture and spoon into prepared baking dish.
In a small bowl, combine Parmesan, breadcrumbs, lemon zest, parsley and remaining 1/2 teaspoon rosemary. Season with salt and pepper. Sprinkle mixture over the casserole. (The casserole will keep, covered, in the refrigerator for up to 2 days.)
Bake casserole for 20 to 30 minutes, or until bubbling. Let stand for 5 minutes before serving.