1. Preheat oven to 350 degrees. Melt butter in a 10-inch skillet over medium heat.
2. Add onions and sauté until soft, about 4 minutes. Remove from heat.
3. Peel and thinly slice potatoes.
4. Place a layer of potato slices in the bottom of 4-qt. casserole dish.
5. Add about one-third of the butter onion mixture, then 4 oz. of ham. Sprinkle liberally with salt and pepper.
6. Continue to repeat this step for three additional layers with no onion butter mixture on top level, and sprinkle salt and pepper on each layer.
7. Mix the milk with the sour cream, stirring well. Pour evenly over entire dish. Place casserole in oven and bake for 1 hour.
8. Remove and add cheese to top of casserole and bake for additional 15 minutes or until cheese begins to brown.
9. Remove from oven and allow to “rest” at least 10 minutes before serving.
10. Sprinkle green onions on top of casserole and enjoy.