Using a mandoline, slice potatoes thinly. Don't worry about making layers of each potato. If they are thin, you will put layer potatoes some on top of the other for about three layers between bechemel sauce.

Place potatoes in 1 cup milk and enough water to cover potatoes by one inch and gently boil until potatoes are almost done. Drain. Save some of the starchy water.

Slice onions very thinly. Melt butter in frying. Soften onions (do not allow to turn color) in butter. Set aside.

Season flour with nutmeg, salt, pepper, thyme,

Melt butter in heavy bottomed pan. When it's almost melted, add flour and stir until incorporated. Add 1/4 cups starchy water and four cups milk to flour mixture to make roux.

Bring roux to boil, continue stirring until roux is thick and creamy. Take off stove.

Layer potatoes, salt, pepper, onion, bechemel sauce, making sure to save some bechemel for top.

Bake at 325 degrees for approximately one hour. I baked at 300 degrees F for 90 minutes. Potatoes browned perfectly. I did this because I had a prime rib in oven at 300 and couldn't turn oven up.

Allow to cool about 10 minutes before serving. Potatoes will be very hot.