Preheat oven to 375 degrees
Cut squash in half lengthwise and scoop out seeds.
Lightly brush cut sides with 1 tsp oil and place, cut side down onto a parchment-lined baking tray.
Bake in the oven or 1 hour or until the squash is tender when pierced with a fork.
Remove from oven and let cool to room temperature.
Cook lasagna noodles according to package directions.
Meanwhile, heat remaining tsp oil in a large saute pan over medium heat
Add onions, and saute for 3 minutes, until onions begin to soften.
Reduce heat o medium-low and continue to cook stirring frequently, until onions are very soft and golden brown.
Scoop out the flesh of the squash into the bowl of a food processor.
Add ricotta, nutmeg, egg whites, fresh sage, salt, pepper, and parmesan and process until smooth.
Transfer mixture to a large mixing bowl and gently fold in the caramelized onions.
Assemble lasagna, cover the bottom of the pan with about 1/2 a cup of the marinara sauce.
Top with a single layer of lasagna noodles, overlapping slightly, then 1 cup butternut squash mixture and 1/2 cup of mozzarella.
Repeat layer 2 more times, starting with the 1/2 cup marinara sauce, layer of noodles, remaining butternut squash, and remaining marinara sauce.
Cover the dish with foil, place onto a baking tray, and back in the oven for 20 minutes.
Remove foil, top lasagna with reserved mozzarella, and walnuts and bake for an additional 5 minutes.
Serve and enjoy.
