Potato and Tomato Gratin

Summer's coming to an end which means that local red fresh tomatoes are, too :(.  If you make one more dish with your fresh tomatoes this summer I highly suggest this one.  The time needed to caramelize the onions and bake the dish are a bit long for a weeknight (unless you prep and assemble in advance) but it's definitely worth it.  This was a great departure from traditional potato gratin.  Really fresh and lighter in taste and texture.  I'll make it again when tomatoes are at their best again next year.

Two Years Ago: Finn Nisu Bread - A tribute to my husband's heritage

Potato and Tomato Gratin
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Ingredients

• 5 tablespoons extra-virgin olive oil
• 6 cups thinly sliced onions (about 1 1/2 pounds)
• 1 tablespoon thyme leaves, divided
• 1 tablespoon unsalted butter
• 1 1/4 pound Yukon Gold potatoes, peeled or not...whichever you prefer
• 1/2 cup heavy cream
• 2 1/4 pound ripe tomatoes, cut into 1/4" thick slices
• 1/4 cup sliced basil
• kosher salt and freshly ground black pepper

Directions

1. Heat a large saute pan or Dutch oven over high heat until hot, then add 3 tablespoons olive oil, the onions, 1 teaspoon thyme, 1 teaspoon salt, and some pepper. Cook for 6 minutes, stirring often, then turn down the heat to medium and add the butter. Cook an additional 10-15 minutes, scraping with a spoon or spatula until the onions start to caramelize. Turn the heat down to low and continue cooking until the onions are a deep golden brown, another 5-10 minutes. Turn the heat off and set aside.
2. Preheat the oven to 350 degrees F.
3. Thinly slice the potatoes (using a mandoline or a sharp chef’s knife) into 1/8-inch-thick rounds. Toss them in a bowl with the cream, 1 teaspoon thyme, 1 teaspoon salt, and some freshly ground black pepper.  Arrange sliced tomatoes on a plate and season with 1 teaspoon salt and some pepper.
4. To layer the gratin, first spread half the caramelized onions in an even layer in a 9×9 inch baking dish. Top the onions with one layer of alternating potatoes and tomatoes (using about half of each), then drizzle with 2 tablespoons cream (from the potato bowl) and 1 tablespoon olive oil. Season with 1/4 teaspoon salt, a healthy pinch of black pepper, 1/2 teaspoon thyme and half the basil.
5. Repeat the layers, making the top potato/tomato layer pretty since this is the top presentation layer of the gratin. Pour the remaining cream (from the potato bowl) and remaining tablespoon olive oil over the gratin and season with 1/4 salt, a pinch of pepper, the remaining 1/4 teaspoon thyme and the remaining basil. Press all the vegetables down with your fingers; the cream will come up through the layers and coat the vegetables evenly.
6. Cover the baking dish tightly with aluminum foil and bake for 1 1/2 to 2 hours, until the potatoes are tender when pierced. Turn the oven temperature up to 450 degrees F, uncover the gratin, and bake an additional 25-30 minutes until the top is bubbly and golden brown. Let cool slightly before serving.

Source: Originally from Sunday Suppers at Lucques by Suzanne Goin as seen on Eggs on Sunday