Scalloped Potatoes

6 to 8 medium potatoes, peeled and cubed or sliced thin
2 TBS butter
1 large onion minced
1 tsp salt
1/2 tsp paprika
1/2 tsp dry mustard
1/4 cup flour
2 cups 1% milk
1 2-ounce bottle diced pimento, drained
1/2 cup finely grated cheese (Optional)

To prepare sauce: melt butter, add onion and cook one minute. Stir in salt, paprika, dry mustard, and flour. Add milk and pimento; cook until thick. stirring constantly. Pour sauce over potatoes, and mix gently. Put in large covered casserole and bake at 325 for 1 1/2 hours or until potatoes are tender. Sprinkle with cheese and serve.