Bring a large pot of salted water to boil. Add tortellini and cook for 1 and ½ minutes. Do not over cook!
Drain and rinse under cold water until cool.
Place tortellini in a large serving bowl. Add in pesto, tomatoes, cheese, and onion. Toss well to coat.
Serve at once, or cover and place in the fridge for up to 3 days.
