On a baking sheet spread out the lentils and pick out any stones. Place in a large sieve and wash under running cold water for about 1 minute. Set aside.
Heat butter in a saucepan over moderate heat. Saute onions with salt and pepper until golden brown. Add garlic, ginger, turmeric and cumin and cook 2 to 3 minutes, stirring occasionally. Add lentils, chicken stock or water and salt and pepper. Bring to a boil, reduce heat and simmer, partially covered, 20 to 25 minutes. Remove from heat and beat the lentils with a wooden spoon or whisk until smooth. Serve dal garnished with coriander.