Preheat oven to 350 degrees. butter a 2 litre casserole. Make a basic whitesauce of butter, flour, salt, pepper & milk. Place half the potatoes incasserole. Cover with half the onion and half the sauce. Repeat layers.
Cover and bake about an hour. (I like to cook it a bit longer, about 1 hourand 15 minutes before uncovering it.) Test to make sure potatoes arealmost, but not quite, fork tender. Uncover and bake 30 minutes longer.
Left over cooked ham layered in the middle makes it almost a one dish meal,and is a good addition...tho I usually serve this with ham to begin with.
Serves 6 but count on serving 4 with it!