In a cast-iron pot or skillet, heat the olive oil over medium-high heat. Once oil is shimmering, add in diced onion, diced carrots, and diced celery. Cook and stir for 7–9 minutes. Add minced garlic, tomato paste, Italian seasoning, dried oregano, dried basil, dried thyme, and red pepper flakes, if using. Season with salt and pepper. Stir until fragrant and combined, another 1–3 minutes. Transfer all this mixture to a 6-quart slow cooker.
Remove any fat from the chicken breasts and cut into chunks—about 3–5 per breast. (This ensures it will cook in the same time.) Place in the slow cooker along with the undrained fire-roasted diced tomatoes, uncooked quinoa (see note 1), bay leaves, and chicken stock. Stir together.
Cover and cook on high for 3–5 hours or until quinoa has “popped” and chicken shreds easily. I don’t recommend cooking on low (see note 3). Once chicken shreds easily, remove from the slow cooker into a separate bowl or plate and shred with two forks. Return to the soup and stir. Remove bay leaves and discard. If desired, add coarsely chopped baby spinach and stir gently to wilt. Taste soup and season with any additional salt or pepper as needed.
Ladle soup into bowls and garnish with fresh basil and lots of freshly grated Parmesan cheese. Enjoy with a side of warm, hearty bread.
Notes
Note 1: Rinse quinoa before adding it to the soup to remove its bitter saponin coating. Use a fine-mesh sieve and rinse under cold water until clear. Many brands are pre-rinsed, so check the package first.Note 2: Serving suggestions: fresh basil, freshly grated Parmesan cheese, and warm, hearty bread.Note 3: When cooked on low, the quinoa finishes a lot quicker than the chicken and veggies. The longer this soup sits or cooks, the more broth the quinoa absorbs. This makes it heartier and less broth-y. If you want a more broth-y soup, watch carefully and turn the heat off the slow cooker when it is your desired consistency. Alternatively, add more broth as desired.Storage: When stored, note that the quinoa will continue to absorb liquid. This soup does not freeze and thaw well.