Spray your slow cooker with nonstick spray - be generous!
Remove any fat from the chicken and cut into smaller pieces. I had 4-5 chicken chunks. Add the chicken to the slow cooker.
Rinse the quinoa and add it to the slow cooker along with the Italian seasoning, dried oregano, pinch of red pepper flakes, garlic, chopped onion, spaghetti sauce, undrained petite diced tomatoes, and chicken broth.
I poured in 4 cups (907g) of chicken broth. Stir everything really well and cover the slow cooker. Allow to cook on high for 3-4 hours. (If you have a really hot slow cooker, maybe even less time and you may have to stir it several times throughout cooking so the quinoa doesn't stick to the edges)
Remove the chicken chunks, shred the chicken with two forks, and add it back to the soup. Stir everything together and add in 1-2 more cups chicken broth if desired (I like it chunkier without the extra broth).
Season with salt and pepper.
Stir in the Parmesan (and mozzarella if desired - just as much as you want) cheese and allow to melt.
Top with fresh basil (and an extra garnish of Parmesan cheese) and enjoy while it's hot!