Potato, fennel, and leek gratin

My friend Candy made this for Christmas Eve, 2022. It’s a creamy, cheesy, delicious gratin! From Fine Cooking, Feb/Mar 2015.

 
Serves 10-12
 
3 T unsalted butter, softened
2-1/2 lbs Yukon gold potatoes, peeled and sliced 1/8 inch thick (about 7 cups)
2-1/2 cups heavy cream
2 gloves garlic, finely chopped
Kosher salt and freshly ground black pepper
1 t fennel seeds, lightly crushed
3 large leeks, white and pale-green parts only, trimmed, halved, and sliced crosswise, 1/2 inch thick
1-1/2 to 1-3/4 lbs fennel bulbs, trimmed, quartered, cored, and thinly sliced (about 5 cups)
8 oz. Gruyère cheese, grated (about 2 cups)
 
Directions
 
Position a rack in the center of the oven and heat the oven to 400 degrees F.
 
Grease the bottom and sides of a 10x15 inch (4 quart) baking dish with 1 T. of the butter.
 
Put the potatoes, cream, garlic, 1-1/2 t salt, and ½ t pepper in an 8-quart pot. Bring to a simmer over medium heat and cook, stirring occasionally, until the potatoes are barely tender when pierced with a fork, about 8 minutes.
 
Meanwhile, melt the remaining 2 T of butter in a heavy-duty 12-inch skillet over medium heat. Add the fennel seeds and stir until fragrant, about 30 seconds. Add the leeks and fennel, season lightly with salt and pepper, and cook, stirring occasionally, until just tender, about 8 to 10 minutes.
 
Gently mix the leeks and fennel with the potatoes. Transfer to the prepared baking dish, evening out the vegetables. Top with cheese, and back until the cheese is deep golden brown, the cream has thickened, and the potatoes are tender when pierced with a knife, 35 to 40 minutes. (If the top is becoming too brown before the vegetables are done, cover the grain loosely with foil).
 
Let cool for 10-20 minutes before serving.
 
Candy’s original notes:  Needed a little more salt.