1. Bring a large pot of salted water to a boil.
2. In a large skillet, heat the oil over medium-heat. Add the onion and cook, stirring, until well browned, about 10 minutes. Add the tomatoes and their juices and bring to a boil. Reduce the heat to medium-low and simmer, stirring occasionally, until the sauce is slightly thickened, about 5 to 10 minutes. Season with salt and pepper.
3. Meanwhile, cook the tortellini until al dente, about 5 minutes. Drain and transfer into a large bowl. Toss with the tomato sauce and basil. Adjust the seasoning with salt and pepper.
4. Sprinkle with the Parmesan cheese and serve immediately.Serving Size: about 2 cups