Chop the onion and garlic. Dice the pumpkin. Heat the oil, sauté the onion and garlic until translucent. Add turmeric, coriander, cumin and chilli flakes and stir-fry for 2 minutes. Add the pumpkin, lentils and 1 liter of hot water and simmer for 20 minutes until the lentils and pumpkin are soft. Puree everything and season with salt and pepper. Let the soup get hot and place on plates. Garnish with the yogurt.