Heat oven to 400 degrees F. Melt the butter in a medium sauce pan. Add the onion and cook for a few minutes (until the onion softens). Stir in the flour and salt and cook until the mixture starts to bubble. Stir in the milk and cook until it boils. Reduce heat to low and stir in the sour cream and cheese. Cook on low until the cheese is melted.

Slice Potatoes into thin slices. Layer half the potatoes in a baking dish. Pour half the sauce over the potatoes and add some cheese. Repeat with remaining potatoes and sauce and add the rest of the cheese on top. Sprinkle paprika on top for color.

Bake for 40-45 minutes at 400 degrees or until bubbly and the top starts to brown.