For the cashew ricotta: Bring a medium saucepan of water to a boil. Boil the cashews until softened, 5 to 8 minutes. Alternatively, place the cashews in a bowl and cover with water. Cover with plastic and soak until slightly softened, for at least 1 hour.
Drain the cashews using a slotted spoon and transfer them to a food processor. Reserve the soaking liquid.
Add the lemon juice, nutritional yeast and salt to the blender and pulse until finely ground. Add about 1/4 to 1/2 cup of the cashew soaking liquid a little at a time until the mixture is smooth, creamy and resembles ricotta cheese.
For the lasagna: Poke the squash all around with a fork, then microwave it in 5-minute increments, rotating in between, until the outside is soft and yields a little when pressed with your fingers, about 15 minutes total. Be careful when rotating and removing the squash, as it will become very hot.
Meanwhile, heat the oil in a large skillet over medium heat. Add the onions and cook until softened, 10 minutes. Add the vinegar and cook until the onions are golden brown, 10 minutes. If the onions dry out, add a dash of water to the skillet.
Remove the chicken sausage from its casings, add it to the skillet and cook, breaking it up in small crumbles with a wooden spoon, until browned and cooked through, 6 to 8 minutes.
When the squash is cool enough to handle, slice it in half, scoop out the seeds, then scrape out the flesh with a fork into a medium bowl. Discard the skin and seeds.
To assemble, layer the spaghetti squash, marinara sauce, sausage-and-onion mixture and cashew ricotta in four 30-ounce glass containers or a 9-by-13-inch baking dish. Top with the sliced basil and serve.