Wash the lentils then add to 5 cups water along with a chopped medjool date. Bring to the boil then turn down the heat and simmer for 30-45 minutes, until lentils are soft. Skim off the skum that forms on the top.
Meanwhile add the mustard seeds to a pan and dry fry until the seeds change colour and smell fragrant. Grind with a pestle and mortar.
Once the lentils are cooked, blend using a hand blender. Add the rest of the ingredients (including the toasted mustard seeds) and simmer for about 15 minutes to allow the spices to infuse the lentils. If the dal becomes too dry, add some more water.