Heat oil in a Dutch oven over medium-high heat. Add onion and carrots; cook 3 to 5 minutes until onions are softened; add mustard seeds, if desired, and cook about 30 seconds for seeds to pop.

Add the tomato paste, garlic, ginger, curry powder, salt and cayenne and cook 30 to 60 seconds until fragrant, stirring constantly. Stir in stock, chickpeas and lentils. Bring to a boil over high heat. Cover and reduce heat to medium-low; simmer 15 to 20 minutes until lentils are tender. Stir a few times to break up lentils for a smoother texture.

Stir in spinach and lemon juice. Serve soup topped with cilantro or stir cilantro into yogurt and dollop onto each serving.