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 in a frying pan on a medium-low heat for 15 minutes, until tender.
3) Peel the potatoes, then thinly slice them by hand or with a mandoline. Mix the sliced potatoes in a large bowl with 480ml of the cream, 200g of the gruyere, salt and pepper. Add the sauteed fennel and onion and mix well.
4) Pour the potatoes into the baking dish and press down to smooth. Combine the remaining 30ml of cream and 50g of gruyere and sprinkle on the top.
5) Bake in the oven 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.