Heat 1 tablespoon olive oil in a large pot over medium-high heat. Add the chopped onion, celery, carrots, and garlic. Cook for 3-4 minutes or until the onion is tender.
Add the shredded cabbage, spinach, diced tomatoes, tomato paste, salt, black pepper, paprika, cumin, and crushed red pepper flakes (if using). Stir to combine and cook for about 5 minutes until the cabbage begins to wilt.
Pour in the vegetable broth and stir well to combine all ingredients. Bring the soup to a boil, then reduce the heat to low and let it simmer for 20-25 minutes, or until the cabbage and vegetables are tender.