Step 1

Preheat the oven to 350 degrees F.

Step 2

Butter the inside of a 10-by-15-by-2-inch (10-cup) baking dish.

Step 3

Remove the stalks from the fennel and cut the bulbs in half lengthwise. Remove the cores and thinly slice the bulbs crosswise, making approximately 4 cups of sliced fennel. Saute the fennel and onions in the olive oil and butter on medium-low heat for 15 minutes, until tender.

Step 4

Peel the potatoes, then thinly slice them by hand or with a mandoline. Mix the sliced potatoes in a large bowl with 2 cups of cream, 2 cups of Gruyère, salt, and pepper. Add the sauteed fennel and onion and mix well.

Step 5

Pour the potatoes into the baking dish. Press down to smooth the potatoes. Combine the remaining 2 tablespoons of cream and ½ cup of Gruyère and sprinkle on the top. Bake for 1 ½ hours, until the potatoes are very tender and the top is browned and bubbly. Allow to set for 10 minutes and serve.