Here's the creamy original version, lightened up just a bit. In Utah, these are commonly made with frozen shredded hash browns and called Funeral Potatoes, because they're traditionally served after funerals. They mostly use canned cream soups, too. Google "funeral potatoes" and you'll find plenty of recipes.
Au Gratin Potatoes
4-6 medium potatoes
1/3 c. butter
1/2 c. flour
3 c. milk (use skim or 1% for a lighter version)
1 t. dried onions
1/2 t. garlic powder
1 t. dried parsley
1 t. salt
1/2 t. black pepper
3 c. shredded cheese (sharp cheddar is best, but you can use a pizza or nacho blend, too)
Slice potatoes into 1/4" thick slices. Place in a saucepan. Add enough water to barely cover them. Bring to a boil. Reduce heat to a simmer. Cover and let cook for 15-20 minutes, just until tender. Drain.
Meanwhile, melt butter in a saucepan. Stir in flour. Cook until bubbly and hot. Add milk a little at a time, whisking smooth between each addition. Add spices. Bring to a boil, stirring constantly. Boil and stir for one minute. Remove from heat. Add 2 c. of the cheese. Stir until cheese is melted.
Layer potatoes in a 2-quart casserole dish. Pour the cheese sauce over the top. Bake at 350° for 30 minutes, until hot and bubbly. Sprinkle with the remaining 1 c. of cheese. Bake another 5-10 minutes until the cheese is melted and slightly browned.
For an even lighter version, reduce the cheese to 1 c. in the sauce and none on top.
If you leave the peels on the potatoes, it boosts the fiber content just a little.
You can add chopped ham, diced onions, diced bell peppers, diced green chilis, or crumbled cooked bacon, if you want this as a main dish instead of a side.