1-1/2 cup onion, finely chopped
Preheat oven to 350° F. Spray a 9" x 13" casserole pan with oven spray.
Sauté onion in oil until tender. Add flour, mustard, salt and pepper. Slowly add milk and broth and cook until thick. Add cheeses and stir to blend, then add sour cream. Combine with potatoes.
Put into sprayed pan and bake, covered, for 35 minutes, and uncovered for 30 more minutes.
This still has fat grams, so don't make a whole meal on this one item, folks.