Potato-Tomato Gratin

Serves 6 to 8

A mandoline makes quick work of slicing the potatoes (tomatoes are better sliced by hand).

Ingredients
2 tablespoons olive oil
2 onions, halved and sliced thin
Salt and pepper
2 garlic cloves, minced
1/4 cup water
1/2 cup pitted kalamata olives, chopped
3 pounds plum tomatoes, cored and sliced 1/4 inch thick
2 pounds russet potatoes, peeled and sliced 1/8 inch thick
2 teaspoons minced fresh thyme
8 ounces Gruyère cheese, shredded (2 cups)


Instructions

1. Adjust oven rack to upper-middle position and heat oven to 400 degrees. Grease 13 by 9-inch baking dish. Heat oil in 12-inch skillet over medium heat until shimmering. Add onions, 1/2 teaspoon salt, and 1/4 teaspoon pepper and cook, stirring frequently, until soft and golden brown, 15 to 20 minutes.

2. Add garlic and cook until fragrant, about 30 seconds. Add water and cook until nearly evaporated, scraping up any browned bits, about 2 minutes. Off heat, stir in olives; set aside.

3. Shingle half of tomatoes in even layer in prepared dish. ­Shingle half of potatoes over tomatoes and sprinkle 1 teaspoon thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper over top. Spread onion mixture evenly over potatoes. Shingle remaining potatoes over onions. Shingle remaining tomatoes over potatoes. Sprinkle remaining 1 teaspoon thyme, 1/2 teaspoon salt, and 1/4 teaspoon pepper over top.

4. Bake, uncovered, for 1 hour. Sprinkle with Gruyère and continue to bake until cheese is browned and bubbly and potatoes are completely tender, 25 to 30 minutes longer. Let cool for 30 minutes. Serve.

TEST KITCHEN TECHNIQUE: LAYER BY LAYER AND ROW BY ROW

Gratins are layered practically by definition. For our Potato-Tomato Gratin, in which the potato slices gently cook in the juices released by the tomatoes, it's critical that each potato layer adjoin a tomato layer.

LAYERS FROM TOP TO BOTTOM
cheese
tomato
potato
onion
potato
tomato