Rinse the split peas and quinoa under cold water and drain.
In the crock-pot, combine the rinsed split peas, quinoa, diced onion, carrots, celery, minced garlic, and diced tomatoes.
In a separate bowl, mix the vegetable broth, olive oil, thyme, cumin, salt, and black pepper. Pour this mixture over the ingredients in the crock-pot.
Stir everything together to ensure the ingredients are well mixed.
Cover the crock-pot with the lid and set it on low for 6-8 hours or high for 3-4 hours. The soup is ready when the split peas and quinoa are tender.
Once cooked, stir the soup and taste to adjust seasoning if needed. Add more salt or pepper if desired.
Serve hot, garnished with fresh parsley if using.