Two-Potato Gratin

Preheat oven to 400 degrees F.

Trim and peel onion, keeping root end intact. Cut onion into thin wedges through root end. In 12-in. skillet, heat oil on medium. Add onion in single layer and sprinkle with 1/8 teaspoon salt. Cook 6 minutes or until golden brown and just tender, turning often; cool slightly.

Meanwhile, peel potatoes; cut into 1/8-in.-thick rounds. Arrange half of potatoes and half of onion in shallow 2-quart baking dish in single layer, overlapping slices. Sprinkle with half of Gruyère. Arrange remaining potatoes and onion on top, creating pattern if desired.

In 2-quart saucepan, whisk half-and-half into cornstarch to dissolve. Add margarine and 1/4 teaspoon each salt and black pepper. Heat on medium-high until bubbles begin to form around edge, whisking. Pour over potato mixture. Cover with foil; bake 30 minutes. Uncover, sprinkle remaining Gruyère on top, and bake 25 minutes or until golden and bubbling.

Cool slightly to serve warm, or cool to room temperature, cover, and refrigerate up to overnight. Reheat, covered, at 350 degrees F for 40 minutes or until hot.