3 lbs sweet potatoes, peeled, quartered lengthwise, and cut into ½-inch slices
2 Tbsps olive oil
freshly ground black pepper
3 Tbsps butter, plus more for prepping casserole pan
4 large leeks, pale green and white parts only chopped (and washed well)
½ C chopped fresh sage
4 oz goat cheese
1 C heavy whipping cream
1/2 cup hazelnuts
Preheat oven to 400F. Toast the hazelnuts on a baking sheet for about 8-10 minutes, transfer the nuts to a towel and rub them together to remove the husks. Roughly chop the toasted, husked hazelnuts and set aside.
In a large bowl, toss the sweet potato slices with the olive oil, some kosher salt, and black pepper. Lay the potatoes on a sheet pan in a single layer (use two pans if necessary) and roast in the oven until they begin to brown (about 35-40 minutes), flipping the slices over about halfway through cooking time.
While the potatoes are roasting, melt the butter in a large skillet over medium-low heat. Add the leeks, season with salt and pepper and sauté until, about 10 minutes. Remove from the heat, add the chopped sage and stir to combine. Coat a 11x7 casserole with the extra butter or a butter wrapper. Spread half of the leek mixture over bottom of the casserole. Spread half of the sweet potatoes over the leeks, then crumble half of the goat cheese on top. Repeat this layering of leeks, squash, and cheese.
|Ready to add the cream, hazelnuts, and bake.|
Lower the oven temperature to 375F. Pour the cream evenly over the gratin and sprinkle with the chopped hazelnuts. Bake uncovered until the gratin is heated through and the cream is bubbling, about 35 minutes.
The gratin can be assembled ahead of time; prepare all ingredients and transfer to the casserole dish, but don’t add the cream or hazelnuts. Store in the fridge until you’re ready to bake, then add the cream, nuts, and increase the baking time by 8-10 minutes.