1. In skillet, melt 2 tbsp. of butter. Add onions and cook until softened. Set aside.

2. In a saucepan, melt 6 tbsp. of remaining butter over medium-low heat; gradually whisk in the flour and cook the roux, whisking for 3 minutes. 

3. Slowly add the milk, whisking constantly and bring sauce to a boil. 

4. Simmer the sauce, whisking for 1 minute. Add salt and pepper to taste.

5. Peel potatoes and slice them into 1/8″ thick slices. Peel sweet potatoes and slice them approx. 1/4″ thick (the sweet potatoes will cook faster)

6. In a well-greased casserole dish, spread 1/3 of the sauce. Next, cover the sauce with a layer of the sliced potatoes followed by 1/3 from the onions.

7. Add 1/3 of the cheddar cheese and continue to layer the remaining sauce, potatoes, onions and cheddar in the same manner.

8. Combine bread crumbs with 2 tsp. of butter. Mix well and sprinkle on top of potatoes.   

9.  Bake covered with foil in 400F oven for 25 minutes. LOWER OVEN TEMP TO 375F

10. Remove foil and bake in 375F oven for 30 – 40 minutes more.  Some potatoes take longer to cook, so test them with a sharp knife at the 30 minute mark, then give them another ten minutes or more until they are easy to pierce with a knife).

11. Remove from oven and let stand 10 minutes before serving.