To a pot or pan, add 2 quarts of water, kosher salt (1 tbsp), 1 tbsp sugar, and 2 tbsp apple cider vinegar
Add cut potatoes and bring to a boil over high heat. Reduce to a simmer and cook, stirring occasionally, until the potatoes are tender (~10 minutes)
Drain the potatoes and transfer them to a rimmed baking sheet. Spread them into an even layer and sprinkle them with the remaining 1 tbsp of vinegar
In a large saucepan, heat ghee/oil and once hot, add mustard seed and coriander and allow to sputter for 30 seconds
Add soaked dal and toast/fry until golden. Then add onion, chiles, and ginger, cooking over medium heat until onions are just beginning to glisten and turn translucent
Add 1 tsp sugar, 1 tbsp kosher salt, turmeric, asafoetida, curry powder, and cayenne. Stir to thoroughly mix and simmer another couple of minutes to let the onions release their liquid and soften more fully
Add 2 cups water and stir well, simmering 2-3 minutes or until the mixture thickens a bit
Add the cooked potatoes and stir well, simmering until everything is combined and a light sauce has formed
Optionally, use a potato masher to break down the potatoes a bit more but don't fully mash them
If using peas, mix them in right at the end so they don't get too smashed up