Preheat oven to 375?F.

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

Slice and rinse the leeks. Heat olive oil or butter in a large skillet over medium heat. Saut? the leeks for 8-10 minutes, covering part way to soften. Add the garlic, sage, and a pinch of salt; cook for 2-3 more minutes. Turn off the heat.

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

Grease a 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 with butternut squash, overlapping slightly. Sprinkle with half of the saut?ed leeks. Drizzle with 1/3 cup cream mixture and sprinkle with 1 ounce of grated gruyere.

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

Add the final layer of butternut, overlapping and pressing down so it is even. Drizzle the remaining cream mixture over the top, lightly sprinkle with cheese and walnuts. Press down in the center so the gratin is level.

Cover with parchment first and then foil. Bake for 45 minutes covered. Uncover, check if the butternut is tender using a knife, then bake at 400?F uncovered for 15 minutes until golden.

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