Scalloped Tuna And Potato Casserole
["2 c. sliced, peeled potatoes", "3 Tbsp. margarine", "1/4 c. all-purpose flour", "2 Tbsp. prepared mustard", "1 tsp. salt", "1/8 tsp. pepper", "2 c. milk", "1 (7 oz.) can tuna", "1 c. sliced peeled onions"]
["Cook potatoes in boiling, salted water 10 minutes and drain. Melt margarine; stir in flour, mustard, salt and pepper.", "Add milk gradually while stirring.", "Cook over low heat until smooth and thickened.", "Arrange potatoes, tuna and onions in alternate layers in greased 2-quart casserole dish.", "Pour sauce over and bake at 350\u00b0 for 45 minutes."]
["potatoes", "margarine", "all-purpose", "mustard", "salt", "pepper", "milk", "tuna", "onions"]