The layered gratin is pictured here underneath the flank steak. That is how it is served at The Wooden Spoon. You could, of course, choose to serve it as a side dish instead of under the steak.
2 tablespoons butter
3 tablespoons olive oil, divided
2 large onions, sliced 1/4 inch thick
Pinch cayenne pepper
1 teaspoon kosher salt, divided
1 pound Yukon gold potatoes
1-1/4 pounds tomatoes
1/2 teaspoon ground black pepper
6 ounces Gruyère cheese, shredded (1-1/2 cups)
2 teaspoons chopped fresh thyme
1 cup soft bread crumbs
In large skillet, melt butter and 1 tablespoon olive oil over medium-low heat. Add onions, cayenne pepper, and 1/2 teaspoon kosher salt. Cook, covered, 13 to 15 minutes or until onion is tender, stirring occasionally. Uncover; cook and stir over medium-high heat 3 to 5 minutes more or until onion is golden.
Preheat oven to 375°F. Use mandoline or slicing blade of food processor to slice potatoes 1/8 inch thick. Line baking sheet with paper towels. Slice tomatoes 1/4 inch thick; place on paper towels to drain excess juices.
Lightly oil 2-quart au gratin or baking dish. Place half the caramelized onions in bottom of dish. Arrange half the potatoes over onions, overlapping slightly. Season with some of remaining salt and black pepper, drizzle with some of remaining oil (reserving 2 to 3 teaspoons for topping), and sprinkle with 1/2 cup cheese and a third of the thyme. Arrange half the tomatoes over potatoes. Season with some of salt and pepper, drizzle with additional oil, and sprinkle with half of remaining cheese and half of remaining thyme. Add remaining onions. Repeat with remaining potatoes, tomatoes, salt, pepper, oil, and thyme; sprinkle remaining cheese on top. Cover with foil. Bake 45 minutes.
In small bowl combine reserved oil and bread crumbs; toss to coat. Uncover baking dish; sprinkle crumbs over top. Bake, uncovered, 10 to 15 minutes more or until the topping is golden brown and vegetables are tender. Let stand 10 to 15 minutes before serving. Makes 6 to 8 servings.