Rinse the lentils and place them in a large crockpot. Add the potatoes, diced onions, garlic, ginger, butter, curry powder, turmeric, cumin, sugar, and red pepper flakes. Stir to combine.
Pour the diced tomatoes in with lentils. Use stock or water to cover the lentils. Stir to make sure that the lentils are covered with liquid. Cover and cook on high for 4-5 hours or low for 7-8 hours.
Keep checking the lentil's water level especially if you are cooking it on high. Keep adding water if it looks too dry. We like it to be the consistency between a soup and a stew. So add more or less water base on your desired finish consistency. Stir in the coconut milk and then the cilantro just before serving. Serve over hot rice.