Pre-heat the oven to 350 degrees. In a small sauce pan, melt the butter and slowly stir in the flour. Let it get golden (2-3 minutes) and add the milk. Stir in cheese, season with salt and pepper. Sauce should be very thick.
Cut the potatoes thick (they may fall apart otherwise). Place 1/3 of the sliced potatoes in the pan, pour 1/3 of the cheese over the potatoes, repeat step twice. Put in the oven for 50 minutes at 350 degrees.