Fill a stockpot two-thirds full with salted water and bring to a boil.
In a large skillet, heat oil over medium-high heat.
Brown onion in hot oil.
Add tomatoes (with juice) and bring to a boil.
Reduce heat to medium-low and simmer, stirring occasionally, until sauce is slightly thickened, about 10 minutes.
Season with salt and pepper to taste.
Add tortellini to boiling water and cook al dente, about 5 minutes.
Drain and transfer to a large bowl.
Toss with tomato sauce.
Stir in basil.
Serve Tomato-Basil Tortellini topped with Parmesan cheese to taste.