Pick over the lentils (there's always one little stone per cup) and cook them, then drain them and set aside.
Chop the onion fine, and fry it in oil. If you have time set the heat on low and let it caramelize a bit. Increase the heat, add the cumin seeds let them roast a little, add the turmeric, ginger, mustard, raisins.
Meanwhile bring the rice, 1 tsp salt, and the broth/water to a boil, lower the heat and let it simmer till the water is almost gone. Add the onion mixture, butter, and the lentils to the rice, stir gently but thoroughly. Nestle the date pieces into the rice, put the lid on and let it steam on low heat for 30 minutes.