In a saucepan melt the butter and whisk in flour to make a roux. Cook for about one minute. Add salt, pepper, sour cream and milk. Gradually stir over medium heat to keep the mixture from burning. Stir until thick, about 2-3 minutes, then add parmesan cheese.
Arrange half of potatoes in casserole dish, making sure they overlap. Pour half of the cheese mixture over potatoes, then repeat the process.
Bake covered for 35 to 40 minutes, then uncover and bake additional 15 minutes.
Top with shredded cheddar and cook another 5 - 10 minutes until cheese is melted and bubbly. Make sure potatoes are cooked through. Test by inserting knife - it it goes in and comes out easily and cleanly casserole is done.