Heat the olive oil in a large pot over medium heat.
Add the chopped onion and garlic; sauté until translucent, about 5 minutes.
Stir in carrots and celery and cook for another 5 minutes.
Add the diced potatoes, rinsed lentils, vegetable broth, thyme, cumin, and bay leaf.
Bring to a boil, then reduce heat and let it simmer for 30-35 minutes, or until lentils and vegetables are tender.
Season with salt and black pepper to taste.
Remove bay leaf, ladle into bowls, and garnish with fresh parsley before serving.