In a large pot of lightly salted boiling water, cook the pasta according to package directions.
About 3 minutes before the pasta is done, add the green beans. Cook the additional 3 minutes. Drain, transfer to a large serving dish, toss with 1 tablespoon of the olive oil, and set aside.
Meanwhile, in a large non-stick skillet over medium heat, heat the remaining olive oil and cook the onions until tender, about 5 minutes.
Add the garlic and tomatoes, stirring to combine and heat through. Pour over the pasta, tossing well to mix.
Serve with Parmesan, if desired.