Instructions

1. Preheat the oven to 375°F (190°C). Generously butter a 9x13 inch baking dish to prevent sticking and ensure easy serving.
2. Wash and thinly slice the sweet potatoes using a mandoline or sharp knife, aiming for uniform slices approximately 1/8 inch thick. Place sliced sweet potatoes in a large mixing bowl.
3. Trim the fennel bulb, removing the tough outer layer and stalks. Slice the fennel bulb thinly, creating delicate, translucent pieces that will integrate beautifully with the sweet potatoes.
4. In a separate bowl, whisk together heavy cream, chopped fresh sage, salt, and freshly ground black pepper. This mixture will infuse the gratin with rich, aromatic flavors.
5. Layer the sweet potato and fennel slices alternately in the prepared baking dish, creating overlapping, slightly diagonal rows to maximize coverage and visual appeal.
6. Pour the cream and sage mixture evenly over the layered vegetables, ensuring all slices are lightly coated and moistened.
7. Sprinkle the grated cheese over the top, creating a uniform coverage that will melt and form a golden, crispy crust during baking.
8. Cover the baking dish with aluminum foil, which will help steam the vegetables and ensure even cooking.
9. Place in the preheated oven and bake for 35 minutes covered, then remove the foil and bake an additional 10-15 minutes until the top is golden brown and vegetables are tender when pierced with a fork.
10. Remove from oven and let rest for 10 minutes before serving. This allows the gratin to set and makes cutting and serving easier.
11. Garnish with additional fresh sage leaves if desired, and serve hot as a luxurious side dish that showcases the sweet and aromatic flavors of fennel and sweet potato.