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 partway through. Add garlic, sage, and a pinch of salt; cook for 2-3 more minutes. Turn heat off.

Whisk the cream with 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, overlapping slightly. Sprinkle with half of the sautéed leeks. Drizzle with 1/3 cup cream mixture. Sprinkle with 1 ounce of Gruyere cheese.

Add the second layer of butternut, using any misshapen pieces. Scatter with the remaining leeks. Drizzle with another 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, lightly sprinkle with cheese and walnuts.

Cover the pan with parchment, followed by foil. Bake for 45 minutes covered.

Uncover, confirm the butternut is tender. Bake at 400°F uncovered for 15 minutes until golden.

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