Preheat oven to 350 degrees.
Rinse potatoes with water, peel them and then rinse them again.
Slice potatoes very thin.
Finely dice onion until you have ¼ of a cup.
Make cream sauce by heating 3 Tbsp. of butter over medium heat.
Blend in flour, salt and pepper until smooth and bubbly, stirring constantly. If it doesn't bubble quickly on medium heat, turn it up to medium-high heat for a bit.
Remove from heat and stir in milk.
Heat on medium-high until boiling and boil and stir for one minute.
In casserole dish layer ⅓ potatoes, ½ onion and ⅓ sauce. Repeat 3 times with no onion on the top layer.
Bake covered on 350 for 30 minutes, then uncover and bake another hour or longer until soft.