Drizzle the grapeseed oil into a large 6-8 quart pot over medium heat. Once the oil is hot, saute the garlic and onion for about 3-4 minutes or until soft.
Add the chicken broth, chicken breasts, lemon zest, lemon juice and red pepper flakes to the pot. Raise the heat to high, cover, and bring to a boil. Once boiling, reduce the heat to medium, then simmer for 5 minutes.
About 10 minutes before you are ready to eat, add the couscous. Simmer about 5 minutes.
Remove the chicken breasts from soup and shred, and then return them to the pot. Stir in the chopped chives and salt and pepper to taste. Sprinkle about 2 tablespoons of mozzarella cheese on each bowl of soup, and garnish with extra red pepper flakes, to taste.
If you need to reheat the soup later in the week, add 2 cups of chicken broth to the pot before reheating.