Preheat oven to 350 degrees. Cut the spaghetti squash in half lengthwise, and scrape out all the seeds and seed membrane. Put the spaghetti squash cut side up in a 9x13 baking dish. Roast uncovered for about 40 minutes. Remove from the oven and allow to cool for at least 10 minutes.
While the spaghetti squash is cooling, make the sausage marinara sauce. Heat a medium size skillet over medium high heat. Add 1 teaspoon olive oil. Then snip the casing of the italian sausage and squeeze the meat into the pan. Break it up with a spoon, and cook for about 5 minutes. Then add the onion to the sausage, and cook another 5 minutes. Then add the minced or grated garlic, and cook for 1 minute. Then add the marinara sauce, and turn the heat to low.
In a small bowl, make the spinach ricotta mixture by combining the ricotta cheese and spinach. Stir to combine, and season with a pinch of salt and pepper.
Once the spaghetti squash are cool enough to touch, scrape the noodles out with a fork, and transfer the noodles to a medium size mixing bowl.
Turn oven up to 425 degrees.
To make the lasagna in the squash, layer the empty squash as follows: marina sauce, 1/4 of spaghetti squash noodles, marina sauce, ricotta cheese, mozzarella cheese, 1/4 of the spaghetti squash noodles, marina sauce, ricotta cheese, mozzarella cheese, marinara sauce, mozzarella cheese. Repeat with the second spaghetti squash. Bake them uncovered in the same baking dish for 15 minutes, or until the cheese is melted and golden brown. Allow to cool for 5 minutes before eating.