To bake onions, assemble recipe in 8-inch square ( 1 1/2-quart ) baking dish; cover with foil. Bake at 450°F. following procedure and times as listed in recipe.
"Au gratin" is a French expression that refers to foods garnished with bread crumbs, cheese and butter, and browned in the oven. "Gratin" refers to both the food and the baking dish. Gratin dishes have low sides and large surface areas to maximize browning.
Serve these mild onions with grilled steak.