Step 1

In a large bowl, soak the lentils for 10 minutes. Drain and rinse well, and then transfer the lentils to the bowl of the slow cooker.

Step 2

Combine the serrano chiles, chopped onion, garlic, ginger, curry powder, cumin, turmeric, chili powder, and sugar in a food processor. Process until the mixture becomes a paste and is well-combined. Transfer the mixture to the slow cooker. Stir in 6 cups of water and the diced tomatoes.

Step 3

Cover the slow cooker and cook on high for 4 hours or low for 8 hours, or until the lentils are soft. Season very generously with salt, stir well, and adjust remaining seasonings, to taste. Gently mash about half of the lentils with a mixing spoon.

Step 4

Stir in the butter, if desired, and the heavy cream or coconut milk, a bit at a time, until reaching the desired richness and consistency. Mix in the cilantro, saving some to top.

Step 5

Serve warm with freshly squeezed lemon, basmati rice, and naan.