In the oven, raise the oven rack so it’s approximately 5 to 6 inches from the broiler, usually the second-highest position. Preheat oven broiler.
Wash hands with soap and water.
Do not rinse raw chicken. Arrange chicken on a parchment-lined baking pan. Season with salt and pepper.
Wash hands with soap and water after handling raw chicken. Wash counters after handling raw chicken.
Broil the chicken for 6 to 8 minutes on each side until internal temperature reaches 165 °F on a food thermometer. Remove from oven and set aside to keep warm.
While the chicken is in the oven, cook quinoa according to package directions.
Scrub the garlic and red onion with a clean vegetable brush under running water. Peel and mince the garlic, and thinly slice the red onion.
Gently rub the grape tomatoes and basil leaves under cold running water. Cut the grape tomatoes, and slice the basil.
If the spring mix greens weren’t prewashed, gently rub them under cold running water. Dry well on a clean towel.
In a large skillet over medium heat, add the oil. Then add the onions in the oil for and cook about 5 minutes, stirring occasionally. Stir in the garlic and cook for 1 minute.
Stir in tomatoes and balsamic vinegar. Cook until vinegar is reduced by half.
Add the chicken to the skillet and top with sliced mozzarella. Cover and cook until mozzarella is melted, about 4 to 5 minutes.
Stir in basil and spoon over chicken breasts.
To make the salad, toss spring mix greens and quinoa together. Arrange mixture on a plate, then top with chicken breast, tomatoes, onions, and balsamic reduction.