Rinse the quinoa thoroughly under cold water and set aside.
In a large skillet, heat olive oil over medium heat. Add diced onion, sliced carrots, and sliced celery. Sauté for 5 minutes until vegetables begin to soften.
Transfer the sautéed vegetables to the slow cooker.
Add the minced garlic, rinsed quinoa, chicken breasts, chicken broth, dried thyme, dried oregano, salt, and black pepper to the slow cooker.
Cover and cook on low for 6 hours, or until the chicken is fully cooked and quinoa is tender.
Remove the chicken breasts from the slow cooker and shred them using two forks.
Return the shredded chicken to the slow cooker and stir in the chopped fresh spinach and lemon juice.
Cover and cook for an additional 10 minutes on low until the spinach is wilted.
Taste and adjust seasoning with additional salt and pepper if desired.
Serve hot and enjoy your hearty slow cooker chicken quinoa soup.