Preheat the oven to 350 degrees Fahrenheit, and butter a 10-by-15-inch baking dish. Remove the stalks from two small fennel bulbs, then cut each bulb in half lengthwise. Remove the cores, then slice each coded bulb in thin slices crosswise. This should provide approximately 4 cups of sliced fennel.
Heat 1 tablespoon unsalted butter and 2 tablespoons olive oil in a skillet on medium-low heat. Add the sliced fennel and one thinly-sliced yellow onion and saute for 15 minutes or until tender.
Peel and thinly slice four large russet potatoes. Place them in a large bowl and add 2 cups heavy cream, 2 cups grated Gruyere cheese, 1 teaspoon kosher salt and 1/2 teaspoon ground black pepper. Stir these ingredients together, then add the cooked fennel and onion slices and mix well.
Pour this mixture into the buttered baking dish. Press down to smooth the top, then combine 2 tablespoons heavy cream and 1/2 cup grated Gruyere cheese to make a topping. Pour the topping mixture over the top, and bake for 1 1/2 hours. Allow the cooked dish to set for 15 minutes before serving.Learn more about Vegetables