Prepare tortellini according to directions. Drain well. Place in a large bowl. Toss with ¼ cup olive oil.
In a blender or food processor, process herbs, remaining ¼ cup olive oil, garlic, balsamic vinegar, salt & pepper.
Toss dressing over tortellini and add the onions and put in the refrigerator until ready to serve.
Just before serving, add your tomatoes and toss gently.