Prepare vegetables: Cut cherry tomatoes in half, slice cucumber in ½" quarters and slice Kalamata olives in half. Thinly slice red onion and keep in a dish of cold water until ready to use to remove some of the sharp flavor.
Cook tortellini according to directions. Bring 4 quarts of water to a rolling boil, add pasta and cook until internal temperature reaches 165° F or approx. 2½ - 3 minutes. Quickly cool under cold running water and set aside.
Fold together the pasta, vegetables and Greek dressing. Finally, gently fold in the feta cheese and mix evenly.
Cover and refrigerate for at least 1 hour for best results.
Before serving, add salt and pepper to taste. If made far in advance, you may want to add another few tablespoons of dressing as pasta may have absorbed excess.
Notes
With pasta salads I often prefer to use prepared salad dressings as they have gums and stabilizers that help cling to the pasta and give it a nice sheen.
If you would like to make your own dressing instead, 2:1 oil to red wine vinegar is a good start. You could add a teaspoon of Dijon mustard and/or mayo to help hold the dressing together and coat the pasta.
Add grilled chicken or shrimp and make it a complete meal.