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2 small fennel bulbs
1 yellow onion, thinly sliced
2 tablespoons olive oil
1 tablespoon butter (plus more for greasing pan)
2 pounds russet potatoes
2 cups plus 2 tablespoons heavy cream
2 1/2 cups grated Gruyere cheese (about 1/2 pound)
1 teaspoon kosher salt
1/2 teaspoon freshly ground black pepper
1. Preheat oven to 350°F. Butter inside of a 10 x 15 x 2 baking dish.
2. Remove the stalks from the fennel and cut the bulbs in half lengthwise. Remove the cores and thinly slice the bulbs crosswise. Saute the fennel and onions in the olive oil and butter on med-low heat until tender.
3. Peel the potatoes, then thinly slice them by hand or on a mandoline. Mix sliced potatoes in a large bowl with 2 cups of cream, 2 cups of Gruyere, salt and pepper. Add the sauteed fennel and onion and mix well.
4. Pour the potatoes into the baking dish. Press down to smooth the potatoes. Combine the remaining 2 tablespoons of cream and 1/2 cup of gruyere and sprinkle over the top.
5. Bake for 1 1/2 hours, until the potatoes are very tender and the top is browned and bubbly. Allow to set for 10 minutes and serve.
6. Can be made ahead--cool baked dish to room temp, cover, then refrigerate. Reheat at 350°F, covered, until heated through, about 30 minutes.