Combine the lentils, 6 cups water, the tomato, ginger, turmeric, cayenne and 1 1/2 teaspoons salt in a saucepan over medium-high heat. Bring to a simmer and cook, vigorously whisking occasionally, until the lentils are broken up and the mixture is thick, about 45 minutes.
Melt the butter in a small skillet over medium heat. Add the garlic and cook 1 minute; stir into the lentils along with the cilantro and lemon juice. Season with salt.