Preheat oven to 350. Lightly grease/spray a casserole dish; set aside.
Cook noodles according to package directions; drain.
While noodles are cooking, melt butter in a saucepan and saute the onions until tender. Stir in the flour until absorbed. Slowly add the milk and salt, stirring constantly. Cook and stir until thickened.
Combine the tuna, cheese, peas, mushrooms, tarragon, paprika and pepper to white sauce; stir well.
Place half of the cooked noodles on the bottom of the prepared casserole dish. Top with half of the creamy tuna mixture. Top that with the remaining half of the cooked noodles -- then the remaining half of the creamy tuna mixture.