Melt butter and saute onion for about 3 minutes.
Add the flour, salt and pepper. Cook and stir constantly until it's bubbly. Remove from heat.
Stir in the milk. Return to heat and heat until boiling, stirring constantly. Boil 1 minute and then remove from heat.
** Number of potatoes will depend on their size. Just start chopping and throw them in the 9x13 pan until the pan is about 3/4 full.