In a medium (2-quart) saucepan, cover the lentils with water by two inches. Let soak for one hour.

Drain and rinse the lentils, then return them to the saucepan, along with three cups of water. Bring up to a simmer over medium heat, then turn down to low and let simmer, covered, just until the lentils begin to fall apart, about 20 minutes. Turn off the heat and set aside.

While the lentils are simmering, heat the oil in a medium (11-inch) skillet over medium-high heat. When the oil begins to shimmer, add the cumin seeds and toast until brown and fragrant, about one minute.

Add the onion, garlic, and jalapeño to the skillet, and sauté until the onions are well-browned, about 20 minutes.

Add the tomato to the skillet, along with the coriander, turmeric, and cayenne pepper. Sauté another five minutes, until tomatoes are beginning to soften and there is no liquid on the bottom of the pan.

Add the sautéed mixture to the pot with the lentils. Add the butter and salt, then process with an immersion blender until smooth. Turn off the heat and stir in the cilantro.

Serve alongside steamed long-grain rice and cucumber raita for a vegetarian meal, or as a soup course if desired.

In a small mixing bowl, stir all of the ingredients together until thoroughly combined.