Butter an 11x7-inch (2 quart) baking dish and set aside. Slice the scrubbed potatoes to 1/8 inch thick circles. Place the potatoes in a large saucepan or Dutch oven; add water to cover by 1 inch. Cook until tender and cool slightly.
Preheat the oven to 350°.
In a 2 1/2-quart saucepan, melt the butter over medium heat. Add the onion; cook 1 minute. Stir in the flour and salt. Cook until the mixture bubbles. Gradually stir in the milk and cook until the mixture boils. Reduce the heat to low. Stir in the sour cream and cheese. Cook on low until cheese is melted, stirring constantly. Layer half of potatoes in baking dish. Pour half the sauce over the potatoes. Repeat with the remaining potatoes and sauce. Sprinkle the potatoes with paprika.
Bake for 30 to 35 minutes or until bubbly and the top starts to brown.