Preheat the oven to 400°F.
Tear the naan into strips and arrange on a baking sheet. Brush with oil, sprinkle with salt and pepper and roast until crisp, 8-10 minutes.
Melt the butter in a large saucepan over medium heat. Add the carrot, celery, onion, garlic, and serrano and cook, stirring regularly, until softened.
Stir in the cumin and cook for 1 minute.
Stir in the lentils, vegetable stock, salt, pepper and ½ bunch chopped cilantro. Bring to a simmer and cook for 45 minutes to an hour, until the lentils are tender, stirring occasionally to prevent burning. Adjust seasoning as necessary.
Place the mint yogurt ingredients in a blender and blend until smooth. Taste and adjust seasoning.
To serve, divide the lentil soup between bowls and garnish with mint yogurt, crispy naan, chopped cilantro and green onions.