Pommes de Terre au Gratin

6 medium potatoes*
1/4 cup butter
1 medium onion, finely chopped (1/2 cup)
1 tbsp flour
1 tsp salt
1/4 tsp black pepper
2 cups milk
8 oz roughly grated Cheddar cheese (2 cups)
1/4 cup bread crumbs

1. Scrub potatoes, rinse, and cut into thin slices.
2. Melt margarine in saucepan over medium heat.
3. Add onion to butter and cook until tender, about 2-3 minutes.
4. Stir in flour, pepper and salt.
5. Remove from heat and add milk and 1 and 1/2 cups cheese.
6. Return to heat and stir until boiling. Allow to bubble for 1 minute.
7. Layer potato slices in a large baking dish.
8. Pour cheese sauce over potatoes.
9. Bake in preheated 360 degree oven until lightly browned, 45-60 mins.
10. Mix breadcrumbs and remaining cheese, and sprinkle this topping over the potatoes. Sprinkle with paprika.
    
11. Return dish to the oven for another 15-20 minutes, until cheese is melted and top is golden brown.
    

*I used Idaho potatoes.