In a large skillet, heat the olive oil over medium-high heat and saute the onion until fragrant and translucent. Add the Italian sausage meat, breaking it up with the back of a wooden spoon and brown it until no longer pink, about 10 to 12 minutes. Drain the excess fat from the sausage meat, and set aside to cool.
In a large mixing bowl stir together the drained and squeeze-dried spinach, ricotta, 1 1/2 cups Parmesan, eggs, cooled Italian sausage and onion mixture, salt, and freshly ground black pepper, to taste.
Place the oven rack in the center of the oven and preheat the oven to 400 degrees F.
In a large pot of salted boiling water, cook the fresh pasta rectangles for 1 minute and remove with a slotted spatula. Set aside.
In a 9 by 13-inch baking dish, spread 2 cups tomato sauce. Working with 1 pasta rectangle at a time, spread 6 tablespoons filling along one edge and roll the pasta sheet up tightly, leaving the ends open. Repeat until you have 8 cannelloni.
Arrange the prepared cannelloni, seam-side down, in a single layer in the baking dish. Spoon the remaining 2 cups tomato sauce evenly over the cannelloni. Sprinkle the remaining 1/2 cup grated Parmesan over top.
Bake the cannelloni for 20 minutes and let it stand for 10 minutes before serving.
Serve the cannelloni with extra tomato sauce from the baking dish and garnish with fresh basil.