Melt butter in a large pan over low heat

Blend in flour, salt and pepper, stirring constantly

Remove from heat and gradually stir in milk

Return to heat and cook until thickened and bubbly

In a large bowl fold in ham, onion and cheese pour over potatoes

Transfer into buttered 9x13 dish, cover with foil and bake at 350 for 30 minutes

Uncover and bake for an hour

Let stand for 10 minutes before serving