Dice potatoes into cubes. Boil them in a saucepan with water until almost soft and cooked through. Melt butter in another large saucepan over medium high heat. Add chopped onion and cook for a few minutes until golden. Add curry paste, garam masala, curry powder, turmeric, chili flakes, sugar, garlic, ginger and stir fry for 1 minute.
Add the chopped tomatoes and cook for another 2 minutes.
Pour in the lentils, coconut milk and potaoes. Stir gently and let it simmer for 5 minutes. Season with salt and pepper. Sprinkle over chopped cilantro.