Bring a large pot of salted water to a boil. Add the pasta shells and cook according to package directions. Drain the pasta into a colander, then return it to the pot and toss with a few dashes of olive oil. Set aside.
Coat the bottom of a medium saucepan with oil and place it over medium heat. Add the onion and sauté until softened, about 5 minutes. Add the garlic and sauté 1 minute more.
Add the crushed tomatoes, sugar, oregano, salt and pepper. Stir a few times to incorporate. Bring the mixture to a simmer and lower the heat. Allow it to simmer, uncovered, for 15 minutes, stirring occasionally. Stir in the basil and remove the sauce from the heat. Taste test and season with additional salt and pepper if you like.
Place the onion, garlic, cashews, milk and lemon juice into bowl of food processor. Blend until smooth, stopping to scrape down sides of bowl as needed. Add the tofu, salt and pepper. Pulse until a thick and slightly chunky, ricotta-like texture is achieved, again, stopping to scrape down the sides of bowl as needed. If some space remains in food processor bowl, add the spinach and pulse until finely chopped and well blended. Otherwise, transfer mixture to a bowl and stir the spinach in by hand. Taste-test and season the mixture with additional salt and pepper to taste.
Preheat the oven to 400º. Coat the bottom of a 9 inch by 9 inch baking dish or 10 inch round oven-safe skillet with about half of sauce.
Stuff the shells with the ricotta mixture and arrange them in a single layer in the baking dish or skillet. Top with the remaining sauce.
Cover and bake for 20-25 minutes, until the sauce is bubbly. Remove the shells from the oven and allow them to sit for 5 minutes before serving.