In a large pot or Dutch oven, heat the olive oil over medium heat. Add the chopped onion and sauté for 3-4 minutes until translucent.
Stir in the minced garlic, diced carrots, and diced celery. Cook for another 5 minutes, stirring occasionally.
Add the diced red bell pepper and sauté for an additional 2-3 minutes until soft.
Pour in the diced tomatoes with their liquid and add the vegetable broth. Bring to a gentle boil.
Stir in the rinsed quinoa, ground cumin, and dried thyme. Reduce heat and simmer for about 20 minutes, or until the quinoa and vegetables are tender.
Add salt and pepper to taste. Stir in the spinach and cook until wilted, about 2 minutes.
If desired, add the lemon juice for an extra zing of flavor.
Serve hot, garnished with fresh parsley.