Servings: 6 to 8

2 to 3 onions, thinly sliced (2 heaping cups) 2 to 3 onions, thinly sliced (2 heaping cups)
2 tablespoons extra-virgin olive oil, more for drizzling 2 tablespoons extra-virgin olive oil, more for drizzling
2 tablespoons butter 2 tablespoons butter
Salt and freshly cracked black pepper Salt and freshly cracked black pepper
2 cloves garlic, minced 2 cloves garlic, minced
2 pounds (about 8) ripe tomatoes, seeded and cut into strips about ⅜-inch wide 2 pounds (about 8) ripe tomatoes, seeded and cut into strips about ⅜-inch wide
2 tablespoons chopped fresh parsley leaves 2 tablespoons chopped fresh parsley leaves
2 teaspoons chopped fresh basil leaves 2 teaspoons chopped fresh basil leaves
1 teaspoon chopped fresh oregano 1 teaspoon chopped fresh oregano
About 2 pounds thin-skinned, waxy potatoes like Yukon Gold, sliced ⅛-inch thick (6 to 7 cups) About 2 pounds thin-skinned, waxy potatoes like Yukon Gold, sliced ⅛-inch thick (6 to 7 cups)
½ cup grated Parmigiano-Reggiano or Gruyère ½ cup grated Parmigiano-Reggiano or Gruyère

Heat oven to 400 degrees and place a rack at the middle level.
In a large saucepan, combine onions, butter and 2 tablespoons olive oil and stir to coat over low heat.
Sprinkle lightly with salt. At a gentle simmer, cook uncovered until onions are softened and lightly golden, 10 to 15 minutes, stirring often. Do not brown; lower the heat if necessary.
Add garlic and cook for 1 minute.
When done, fold in tomatoes and fresh herbs. Remove from heat without further cooking. Set aside.
In a medium-size baking dish, spread a quarter of the onion-tomato mixture. On top, arrange half the potato slices. Add half the remaining onion-tomato mixture. Build one more layer with remaining potato slices and onion-tomato mixture.
Sprinkle cheese on top and drizzle with olive oil.
If the dish is very full, place on a baking sheet. Transfer to oven and bake, uncovered, until potatoes are tender and have absorbed all of the juices in the bottom of the dish, 40 to 50 minutes.
Test by tasting a bit of potato; it should be soft all the way through, with no raw taste. If the top is browning too fast, cover very loosely with a sheet of foil. If the top is not brown enough when the potatoes are cooked, broil briefly until deliciously glazed.
Serve gratin hot or warm. Can be cooled and reheated later the same day, but do not refrigerate.

Can use 1 1/2 teaspoons dried Herbes de Provence in place of fresh herbs. Try adding cheese half way through baking.  Can use 1 ½ teaspoons dried Herbes de Provence in place of fresh herbs. Try adding cheese half way through baking.