Heat the olive oil in a large saucepan over medium heat. Add the onion, celery and carrots and cook until softened, 3 to 4 min. Stir in the garlic, cumin, turmeric and cook until fragrant, about 30 seconds. Stir in the broth and lentils. Cover and simmer until the lentils are tender, about 40 min.
Take soup off heat. Using a potato masher, break up about a third of the lentils. Stir in the cauliflower and simmer for about 8 min. or until tender.
Divide the soup into 4 bowls. Squeeze lemon juice on top and garnish with yogourt, green onion and pepper.
Purée this soup for a smoother texture. It can also be brightened with red lentils.