Preheat oven to 375?F.

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

Slice and rinse the leeks. Heat oil or butter in a large skillet over medium heat. Saut? the leeks for 8-10 minutes, covering part way through. Add garlic, sage, and a pinch of salt, and 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. Add 1/4 cup water to the bottom. Layer the 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 gruyere cheese.

Add a second layer of butternut squash using any misshapen pieces. Scatter with remaining leeks, drizzle with 1/3 cup cream mixture, and sprinkle with gruyere cheese.

Add the final layer of butternut, pressing down so it is level. Drizzle with the remaining cream mixture, lightly sprinkle with cheese and walnuts. Shake the pan gently and press down in the center.

Cover the dish with parchment and then foil. Bake covered for 45 minutes. Check if butternut is tender, then uncover and bake at 400?F for 15 minutes until golden.

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