Spread lentils on a cookie sheet and pick out any stones or clumps of dirt. Place in a large bowl and wash under cold, running water until water runs clear, about 10 minutes. Drain in a colander.
Heat 2 tablespoons of the butter in a medium saucepan. Saute onions with salt and pepper until golden brown. Add garlic and ginger, and cook 2 to 3 minutes, stirring occasionally. Add dal and chicken stock or water. Bring to a boil, reduce to a simmer, and cook, covered, 20 minutes. Just before dal is ready, heat the remaining 2 tablespoons butter in a small skillet. Add the neem leaves and black mustard seeds and fry until leaves are crisp and seeds are popping, about 1 minute. Stir fried leaves and seeds into dal and serve immediately. Dal keeps well for 2 to 3 days. Reheat before serving.
c.1997, M.S. Milliken & S. Feniger, all rights reserved