Heat a large saute pan over medium-high heat. Add the olive oil and heat. Add the garlic and cook for 1 minute. Add the chicken and cook about 4 minutes per side, or until cooked through and juices run clear. Remove the chicken and set aside. To the same pan, add the spinach and cook just until wilted, about 1 to 2 minutes. Remove from the pan and set aside. Lower the heat to medium and add the balsamic vinegar and chicken broth to the pan and stir, scraping the bottom of the pan to remove any browned bits. Add the tomatoes, bring to a simmer and cook 3 to 5 minutes.
Place the couscous in a serving bowl. Top with the spinach, chicken and balsamic-tomato sauce.