Place sliced potatoes in a shallow, broilerproof baking dish.  I like to use an oval shaped au gratin dish.

Pour in one can of evaporated milk.

The first layer will be sliced or chopped onions,  and then the grated cheese.  Next place pats of butter/margarine on top.

Add the bread crumbs if desired or you can also melt some of the margarine and toss into the bread crumbs before adding.

Bake potatoes au gratin at 400° for about 45 minutes. The top should be light golden brown, and the sauce bubbly and thicker.