Be sure to rinse all lentils thoroughly, until the water runs quite clear.
In a large dutch oven (or other large pot), add in all the ingredients for the dal and bring to a boil.
Once dal reaches a boil, turn the heat down and simmer for 30 minutes. Stir occasionally, mashing lentils down to create a thick (but still quite liquid) mixture.
In a separate skillet, heat oil over high heat. Add in mustard and kalonji seeds, allowing them to sizzle for 5-10 seconds. Add in the onion and the rest of the spices, stirring until the onion starts to brown.
Pour onion/spice mixture over dal, and stir to incorporate completely. Add salt to taste.
Simmer for a further 15 minutes to allow the flavors to marry.