Soak brown and red lentils in ample cool water for 1 hour to overnight. Drain and rinse.
Place drained lentils into a large saucepan or Dutch oven. Pour in the water, then add garlic, salt, coriander, cayenne, and turmeric. Bring to a boil over high heat, then reduce heat to medium-low, cover, and simmer until the lentils are tender, about 30 minutes.
While the lentils cook, melt the clarified butter in a skillet over medium heat. Stir in the onions, and cook, stirring often, until they turn golden brown. Stir in the cumin, and cook until fragrant, about 1 minute.
Stir the onions and milk into the lentils; cook for another 6 to 8 minutes. Sprinkle with cilantro to serve.