Place the onion, carrots, sweet potato, celery, garlic, diced tomatoes, vegetable broth, thyme, rosemary, salt, and pepper into the slow cooker.
Stir in the rinsed quinoa until everything is combined.
Cover and cook on low for 6 to 7 hours or on high for 3 to 4 hours until the vegetables are tender and the quinoa is fully cooked.
Add the chopped kale during the last 30 minutes of cooking so it softens but keeps its color.
Stir the soup once more, adjust salt and pepper if needed, and serve warm.