In a small bowl, combine the coriander powder, cumin powder, turmeric, Kashmiri chili powder, kasoor methi and salt. This is your spice mix. It goes in after the garlic ginger paste.
Heat 4 tbsp vegetable oil in a pot over medium heat until it shimmers.
Add the garlic ginger paste. Cook until the garlic ginger paste stops spluttering.
Turn the heat to medium low. Add the spice mix. Stir continuously until it starts to smell really good. This takes around 30-40 seconds. Watch it carefully. If you burn the spices at this point you have to start over. This is called blooming the spices. It is one of the secrets to Indian cooking. Works well in Mexican cooking as well.
Add half the onion paste. Stir to combine. Cook over medium heat for about 2 minutes. It is going to splatter. Be warned.
Add the remaining onion paste. Cook another 2 minutes or so.
Turn the heat to medium low. Add the potatoes and chickpeas. Cook for around 20 minutes. The potatoes should be nearly done.
Now add the chicken and stir. Cover and simmer until the chicken is done. It's done when you measure the internal temperature and it says 170F. That takes about 12-15 minutes. An instant read thermometer is a handy thing to have here...
If the curry is a bit thick add a bit of water or chicken stock and stir. Add the diced and cilantro and lemon juice. Let the chicken masala simmer another minute or so. Taste for salt and adjust as needed.
Garnish with a bit more cilantro if desired.