Set potatoes to boil in a pressure cooker. We want the potatoes to be completely cold (fridge cold is best!) so you can boil them a day ahead as well.
Pick leaves from the long, fibrous methi stems. This step takes time. Then, wash and soak the leaves under running water 2-3 times to remove all the dirt. I let the leaves soak for 5-8 minutes in every wash. Repeat 2-4 times or as needed till you see no dirt in the soaking water. Make sure that the leaves have no mud on them.
On a clean kitchen towel, spread the washed & drained methi to completely air dry for at least 30 to 45 minutes. If you are in a hurry, use paper towel to press down and absorb as much moisture as possible. Ensure that the leaves are totally dry once you are ready to cook else the sabzi will come out watery. Once dry, using a sharp knife, chop the methi leaves. Peel the potato skins and cube into 1 to 1.5 inch cubes.
In an iron karahi or heavy cast iron pan, heat up the mustard oil on medium until slightly smoky and the raw smell goes away. Temper the oil with methi dana and cumin seeds. Wait a few seconds to crackle.
Turn the heat to low and immediately add the chopped garlic, hing and dried chillies. Wait till the garlic changes color to light brown and the dried chillies swell, about 8-10 seconds in hot oil. Take utmost care that the garlic does not burn. You can take the pot/kadai off the stove for few minutes if needed.
Reduce the stove to low and next add the potato and sprinkle the turmeric(if using). Stir around for a minute or so and get the potatoes started in oil. Note- If you are using raw potatoes, add a splash of water, cover the kadai and cook potatoes to fork tender before adding methi leaves. Add the chopped methi leaves. Stir to combine. The methi leaves will wilt down in 1-2 minutes and you will see bit water of the methi separating. Let cook for 2-3 minutes on medium low heat.
Sprinkle the salt next. Stir to combined. Now, cover the kadai for 3 -4 minutes or so and let cook until the potatoes are fully done.
Once done, methi will be a darker shade of green and will stick to potatoes and potatoes will be fully soft.
Put off the heat, sprinkle amchoor, mix gently (so that potatoes dont break) and let sit for at least 20 minutes before serving.
Reheat on low and serve.