Cook quinoa as directed on package.
While quinoa cooks, season chicken with salt and pepper. (You may also pound out chicken so that it is all the same thickness, making it cook more evenly). Spray a skillet with cooking spray and heat over medium. Add chicken and cook each side 4-5 minutes, browning until no longer pink in the center.
Chop cucumbers, tomatoes, and bell peppers, then add to a large bowl. Toss with olive oil and vinegar. Split in half, placing half the mixture into a separate, sealable container. This will be used for another side later in the week. The other half of the mixture can be tossed with the cooked quinoa.
Add an even amount of veggies and quinoa to each of four plates. Sprinkle each with feta and slices of avocado.
Cut cooked chicken into strips, then add to plate with veggies and quinoa. Serve with clementines on the side.