Cook noodles, drain. Meanwhile, press excess moisture from thawed spinach; set aside.
In saucepan cook chopped onion in 3 TBS. butter til tender. Blend in flour, dry mustard, and salt. Add milk all at once. Cook and stir until thickened and bubbly. Add shredded cheese, stir tip melted. Stir in cooked noodles, thawed spinach, water chestnuts and tuna. Put in a 1 1/2 quart casserole dish. Combine bread crumbs and 1 tablespoon melted butter and sprinkle atop. Bake in 350 oven for 40 to 45 minutes or tip heated through. Makes 4 to 6 servings.