Preheat oven to 375°F.

Peel and thinly slice the butternut squash into 1/8-inch thick rounds. Set aside 12-15 of the 'prettiest' slices for the top of the gratin.

Slice and rinse the leeks. Heat oil or butter in a large skillet over medium heat. Sauté the leeks for 8-10 minutes, covering part way through.

Add the garlic, sage, and a pinch of salt to the leeks, and cook for 2-3 more minutes. Turn heat off.

Whisk the cream with the salt, pepper, nutmeg, and onion powder in a medium bowl.

Grease a large 9 x 13-inch baking dish or 11-12 inch round baking dish. Add 1/4 cup water to the bottom of the pan.

Layer butternut squash in the dish, overlapping slightly. Sprinkle with half of the sautéed leeks. Drizzle with 1/3 cup cream mixture and sprinkle with one ounce of gruyere.

Add the second layer of butternut using up any misshapen pieces. Scatter with the remaining leeks. Drizzle with 1/3 cup cream mixture and sprinkle with grated gruyere cheese.

Add the final layer of butternut, overlapping and pressing down. Drizzle with the remaining cream mixture, sprinkle lightly with cheese and walnuts.

Cover with parchment followed by foil. Bake for 45 minutes covered. Uncover and confirm the butternut is tender using a knife.

Bake uncovered at 400°F for 15 minutes until golden.

To make the crispy sage leaf garnish, heat 1-2 tablespoons oil or butter in a pan, add 8 sage leaves, sauté each side for 30 seconds until crisp, and place on a paper towel to cool. Arrange over the gratin.