In a large pot combine lentils, 1/2 of the diced onion, turmeric, cayenne and garlic. Add just enough water to cover your second knuckle when your finger is resting on lentils. Bring to a gentle boil until froth breaks up and disappears. Cover and let simmer. If the soup gets too thick, add more boiling water.
In a smaller pot, melt oil and saute cumin seed just until fragrant. Add the remaining onion and caramelize. Once onions are browned, add diced tomato and cook until it turns into a paste. Add a pinch of cayenne. Add cilantro leaves & stems. Add 1 tsp sea salt.
Now add the paste to the large pot and mix together. Allow pot to simmer for about 15 - 20 minutes. Garnish with more cilantro. Enjoy!