1
Purée the ginger and garlic in a blender with 1/4 cup water. Set the paste aside.
2
Heat 1/4 cup of the butter or coconut oil in a medium saucepan over medium-high heat. Add the onions and cook, stirring occasionally, until lightly browned, about 20 minutes. Add the reserved paste along with the cayenne, coriander, cumin, and turmeric and cook, stirring occasionally, until the water has evaporated and the paste is light brown, about 5 minutes. Add the dal along with 4 cups water. Bring to a boil over high heat, then reduce to medium-low and simmer, stirring occasionally, until the dal has broken down into a moderately thick purée, about 20 minutes. Season with salt, remove from heat, and transfer to a bowl.
3
Toast black mustard seeds, cumin seeds, and chiles in a small skillet. Place over medium-high heat and cook, until fragrant. Mix into dal before serving.
Recipe source: http://www.saveur.com/article/Recipes/Tadka-Dal-