Heat large pot of water to boiling for pasta. (When it comes to a boil, add salt.).
Meanwhile, heat oil in medium skillet over low heat. Add onion and cook, stirring occasionally, until softened, about 5 minutes. Stir in plum tomatoes with juice and the basil. Increase heat to medium and cook uncovered, stirring and breaking up tomatoes, until sauce is thickened, about 15 minutes. Stir heavy cream into sauce. Cook, stirring frequently, until sauce thickens again, about 5 minutes. Season with salt and pepper; keep warm.
Meanwhile, add tortellini to salted boiling water and cook according to package directions. About 2 minutes before it should be done, add spinach and cook until tortellini is ready, about 2 minutes longer. Drain well and transfer to wide serving bowl. Spoon sauce over top, sprinkle with Parmesan cheese, and serve hot.