Prepare the Vinaigrette: Combine all of the dressing ingredients in a large jar or measuring cup. Shake (or whisk) until completely combined. Set aside.
Prepare the Chicken: Season chicken with salt and pepper on all sides, to taste. Heat olive oil in a large skillet over medium heat. Place chicken in the skillet and allow the chicken to cook on one side, undisturbed, for at least 6-7 minutes. Flip, and cook the chicken for about 6-8 more minutes, or until it is nicely browned on both sides and cooked through.
The chicken is cooked through when the internal temperature reaches 180 degrees F, or when juices run clear.
When the chicken is cooked through, remove it from the skillet and place it on a cutting board to rest.
Prepare the Tortellini: Cook the tortellini in a large saucepot according to package instructions. Drain.
While the tortellini cooks, dice the chicken into bite-size pieces.
Assemble the Salad: In a large bowl, gently combine diced chicken, cooked tortellini, spinach, onion, tomatoes, cucumbers, olives, and feta cheese. Pour about ¼ cup vinaigrette over the salad (or more, to taste). Reserve remaining vinaigrette.*
Cover and refrigerate salad until ready to serve. You can prepare this dish up to 1 day in advance. Stir in the chopped fresh basil just before serving.