MIREPOIX: In a cast-iron pot or skillet, heat the 3 tablespoons olive oil over medium-high heat. Once the oil is shimmering, add in the 1 cup diced onion, 1 cup diced carrots, and 1 cup diced celery. Cook and stir for 7-9 minutes. Add in the 2 teaspoons minced garlic, 1/4 cup tomato paste, 1 teaspoon Italian seasoning, 2 teaspoons dried oregano, 2 teaspoons dried basil, 1/2 teaspoon dried thyme, and 1/4 teaspoon red pepper flakes if using. Season with salt and pepper. Stir until fragrant and combined, another 1-3 minutes. Transfer all of this mixture to a 6-quart slow cooker.
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 amount of time.) Place in the slow cooker along with the 2 cans of undrained fire-roasted diced tomatoes, 3/4 cup uncooked quinoa (See Note 1), 2 bay leaves, and 6 cups chicken stock. Stir together.
COOK: 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 2). 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 in 2 cups coarsely chopped baby spinach and stir gently to wilt. Taste soup and season with any additional salt or pepper as needed.
SERVE: Ladle soup into bowls and garnish with fresh basil and lots of freshly grated Parmesan cheese. Enjoy with a side of crusty bread.
STORAGE: When stored, note that the quinoa will continue to absorb liquid. This soup does not freeze and thaw well.