In a large non-stick pan, heat the oil on medium heat.
Add the mustard seeds and curry leaves and fry until fragrant.
Add the minced garlic and ginger and fry until fragrant, for ~30 seconds. If using, add the freshly minced jalapenos (amount depends on preferred level of spice).
Turn the heat on medium-low. Add chopped onions and saute until they start to become translucent.
Add the diced potatoes. Mix to combine. Add a tiny amount of water (2-3 tbsp) and cover the lid for the pan.
Cook covered, stirring occasionally, until the potatoes are tender and cooked but still hold shape. This may take 15-20 minutes.
Half way through, add the turmeric powder, salt, ground cumin, fresh lemon juice, and sugar. Mix to combine and continue to cook the potatoes, covered, until tender. If you feel like the potatoes are starting to stick to the pan and burn, add tiny amounts of water as needed (the moisture from covering the pan should also help).
Once the potatoes are done, uncover the pan, and evaporate any tiny amount of liquid remaining on medium heat.
If needed, adjust the amount of salt, lemon juice, and sugar to taste.
Lastly, add the freshly chopped cilantro, mix, and cook for another few minutes.
Turn off the heat and serve hot as is, or alongside some Naan, fresh roti, or Dosa.
Notes
Seasoning - Depending on the level of acidity of the lemons you are using, you may need to adjust the amount of salt, fresh lemon juice, and sugar at the end to taste.Potatoes - If you end up over-cooking the potatoes and they do not hold shape, it'll still taste great, just have a different consistency.