Heat oil in a large pot over medium heat. Add onion and cook for 4-5 minutes until softened. Stir in garlic and cook for another 30 seconds.
Pour in the petite diced tomatoes and chicken broth. Stir in basil, oregano, and red pepper flakes. Bring to a simmer.
Stir in riced cauliflower and simmer for 8-10 minutes, until tender.
Stir in the cooked chicken and chopped spinach. Simmer another 5-7 minutes until the spinach is wilted and everything is heated through.