Chicken Spinach Artichoke Soup

Name: Chicken Spinach Artichoke Soup
Ingredients: 1 tbsp. extra-virgin olive oil; 1/2 yellow onion, chopped; 2 celery stalks, chopped; 3 cloves garlic, minced; 1/2 tsp. red pepper flakes; Kosher salt; Freshly ground black pepper; 6 c. chicken broth; Juice of 1/2 lemon; 1 1/4 lb. boneless skinless chicken breasts (about 3 large breasts); 1 (14-oz) can artichoke hearts, drained and quartered; 1 c. shredded mozzarella; 1 c. freshly grated parmesan, plus more for garnish; 2 tbsp. heavy cream (optional); 4 c. fresh spinach; 2 tbsp. freshly chopped parsley, for garnish