Bring chicken broth to a boil in a saucepan then add quinoa; reduce heat to a simmer and cover the pan. Simmer until the broth is absorbed, the quinoa is fluffy, about 14-17 minutes.
While quinoa cooks, heat 2 tablespoons of olive oil in a skillet; add onion and cook for 2-3 minutes then add in garlic and continue to cook 30 seconds. Stir in the chicken breast pieces and cook until the chicken is still slightly pink in the middle, about 5 more minutes. Remove the chicken and onion from the pan and set aside.
Pour 2 more tablespoons of olive oil in the skillet and cook and stir the zucchini and tomato until the zucchini is tender, 5 to 8 minutes. Return chicken to skillet and sprinkle with half of the feta cheese, basil leaves, and lemon juice. Cook until the chicken is fully cooked and hot, about 10 more minutes. Serve over hot quinoa. Top with additional feta.
