In a large pot heat about 5 tablespoons of oil. When hot, throw in mustard seeds and sauté until they all pop. Then, add in dals, and dried chilies sauteing until they turn golden brown at which point add the chopped onions.
Sauté until the onions turn golden brownish (add a little more oil, if necessary). Add the diced potatoes and about 3/4-1 cup of water. Mix well then add the remaining seasonings.
Cook the potatoes down over medium-low heat keeping the pot covered and stirring about every 7 minutes until soft.
Notes
*black lentil is usually the whole urad bean or urad dal. The product sold as white lentil is the same lentil with the black skin removed.