Heat the oil in a large saucepan over medium heat. Add the onion and garlic and sauté for 6–7 minutes. Add in chicken, oregano, basil, crushed red pepper, salt, and ground black pepper and sauté for about 5 minutes.
Add in the broth, corn, artichoke hearts, and tomatoes and bring to a boil. Lower the heat, simmer, uncovered, for 20 minutes.
Break up the frozen spinach into small pieces and add to the soup. Simmer 5 minutes until heated through. Add in the lemon juice and remove from the heat.