How to Make It
In a large enameled cast-iron casserole, melt 1 tablespoon of the butter over moderately high heat. Add the onion, season with salt and cook, stirring occasionally, until golden, about 5 minutes. Add the garlic and ginger and stir for 30 seconds, until fragrant. Stir in the mustard seeds, ground cumin, turmeric and cayenne. Add the red lentils and stir to coat with the spices and aromatics. Add 5 cups of water and bring to a boil. Cover partially and simmer over moderately low heat, stirring occasionally, until the lentils break down to a puree, about 25 minutes. (If the lentils become dry before they’re tender, add water 1/2 cup at a time and continue simmering.)
In a small skillet, melt the remaining 2 tablespoons of butter over moderate heat. Add the cumin seeds and curry leaves and stir for 30 seconds, until fragrant. Remove from the heat.
Season the dal with salt and pepper. Transfer to a serving bowl and top with the cumin seed butter. Serve with rice, chopped cilantro, tomatoes and chile, lime wedges and yogurt.
Fresh curry leaves are small, shiny, bright green and fragrant. They are available at South Asian markets and kalustyans.com.