Place the rinsed lentils, vegetable broth, carrots, celery, onion, garlic, diced tomatoes with their liquid, thyme, cumin, bay leaf, salt, and pepper into the slow cooker. Stir to combine everything evenly.
Cover and cook on low for 7 to 8 hours, until the lentils are soft and the vegetables are tender.
About 20 minutes before the stew finishes, rinse the quinoa under cold water. Combine the quinoa and 2 cups water in a medium saucepan. Bring to a boil, then reduce to a simmer, cover, and cook for 15 minutes until the water is absorbed.
Remove the bay leaf from the stew. Fluff the quinoa with a fork. Spoon the quinoa into bowls and top with the lentil stew. Sprinkle with chopped parsley before serving.