Het olive oil in a large stock pot. Add onion and garlic and cook for 3 minutes. Add lemon zest, red pepper flakes and thyme and cook another 3 minutes. Add spinach and orzo, stir to coat, and cook for 1 minute. Add chicken broth, lemon juice and shreddeed chicken. Bring to a light simmer and cook for 12 minutes. In a small bowl, whisk the 2 tablespoons water with the cornstarch until smooth. Whisk mixture into soup and allow the soup to thicken slightly. Serve topped with Parmesan cheese.