Preheat the oven to 350F degrees and butter the inside of a 10 x 15 x 2 inch or equivalent (10 cup) baking dish.
Remove the stalks from the fennel, and cut the bulbs in half lengthwise. Remove the cores and thinly slice crosswise to make approximately 4 cups of thinly sliced fennel.
Saute the fennel and onions in the oil and butter until tender.
Peel, thinly slice, and mix the potatoes in a large bowl with 2 cups of cream, 2 cups of Gruyere, salt, and pepper. Add the sautéed fennel and onions and mix well.
Pour the mixture into the baking dish and press down to smooth the potato tops. Combine the remaining 2 tablespoons of cream and 1/2 cup Gruyere and sprinkle over the top of the potatoes.
Bake for 1 1/2 hours or until the potatoes are just tender and the top is browned and bubbly. Allow to set for 10 minutes and serve.