Bring the 4 cups of chicken broth or stock in a 3 quart pot to a boil. Add orzo pasta, cook for 8 to 10 minutes, until al dente. Remove from the heat, do not drain and cover the pot with a lid.
Heat the oil in a skillet over medium-low heat, and sauté the garlic and chopped onions for about 5 minutes, until the onions are soft. Stir in the chicken, season with salt, pepper and the optional red pepper flakes and sauté for 2 to 5 minutes, until the chicken is lightly browned and the juices run clear. Stir all of the contents of the skillet and the spinach into the pot of broth and cooked orzo. Serve immediately if the broth is still at serving temperature or quickly heat it over high heat till it is of serving temperature. Serve topped with Parmesan cheese, if desired.