Thanksgiving recipe: Butternut squash and potato gratin

Experiment with serving this instead of sweet potatoes.
Butternut Squash and Potato Gratin
Yield: 8 servings
INGREDIENTS
2 tablespoons butter, plus more to grease baking dish
2 large leeks or 3 small leeks, washed, chopped
Kosher salt and freshly ground black pepper
1 teaspoon finely chopped fresh thyme
2 cups heavy cream, plus more if needed
1/2 teaspoon sweet paprika
1/4 teaspoon grated nutmeg
1 butternut squash (with thick neck), about 2 pounds
1 pound Yukon Gold potatoes, peeled
1/4 cup grat