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, then 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 the butternut squash overlapping slightly. Sprinkle with half of the sautéed leeks, drizzle with 1/3 cup of the cream mixture, and sprinkle with one ounce of Gruyere cheese.

Add the second layer of butternut, using up 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 to avoid a dome shape. Drizzle with the remaining cream mixture, sprinkle with cheese and walnuts. Shake the pan and press down to ensure the gratin is level.

Cover the dish with parchment paper (as a barrier) and foil. Bake for 45 minutes covered. Uncover and check the butternut for tenderness using the tip of a knife, then bake at 400 F uncovered for 15 minutes until golden.

To make 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 about 30 seconds until crisp, and place on a paper towel to cool. Arrange over the gratin before serving.