1 medium (3 pound) butternut squash, cut into 1 inch cubes (about 6 cups)
1 large onion, coarsely chopped
2 tablespoons olive oil
½ pound jumbo dried pasta shells
1 ½ cups raw cashews, soaked in water 4 to 8 hours, drained and rinsed (divided)
2 garlic cloves, minced
1 ¼ to 1 ¾ cups unflavored soy or almond milk (divided)
1 tablespoon lemon juice
2 tablespoons chopped fresh sage leaves (about 8 leaves), plus extra for garnish
1 cup chopped walnuts
Salt and pepper
¼ teaspoon nutmeg
Preheat oven to 400°F. Place butternut squash and onion into a large roasting pan or oven safe skillet and add olive oil. Toss to coat. Place into the oven and bake until squash is tender and onions are lightly browned, about 45 minutes, flipping once or twice.
While the squash bakes, bring a large pot of water to a boil. Add pasta and cook according to package directions.
Drain the shells into a colander, return them to the pot, and toss with a few dashes of olive oil to prevent them sticking together.
To make the filling, place 2/3 of the cashews (this will be about 1 1/3 cups after soaking, because they expand a bit) into the bowl of about a food processor fitted with an S-blade. Add garlic, ½ cup of milk, and lemon juice. Blend until the mixture is mostly smooth (it doesn’t have to be perfect). Add half of the roasted squash and onions, and all of the walnuts and sage leaves. Pulse a few times just to mix the ingredients, keeping the mixture a bit chunky and being careful not to over-blend them. Season the mixture with salt and pepper to taste. Transfer the mixture to a bowl.
To make the sauce, place remaining squash and onions into the food processor bowl. Add remaining cashews (just over ½ cup), ¾ cup of milk, and nutmeg. Blend until smooth, stopping to scrape down the sides of the bowl as needed. Thin the mixture with up to an additional ½ cup of milk. It should be saucy and a bit thicker than soup.
Season with salt to taste.
Spoon about 2/3 of the butternut squash sauce into the bottom of a 9 x 13 inch baking dish. Stuff each shell with about 2 tablespoons of the filling, then place into the baking dish. When all shells are filled, cover the dish loosely with foil. Bake until heated throughout, about 20 minutes.
Warm the remaining sauce up on the stove or in the microwave, adding a few additional splashes of milk if it gets too thick. When the sauce is hot, spoon it over the baked shells.
Sprinkle with some additional fresh sage and serve.