If using, chop potatoes into 2 inch cubes and boil. Once softened, drain and set aside.
In a large saucepan or wok, heat 2-3 tbsp oil. Add onions and 1 tsp salt. Stir fry till they start to soften and turn golden.
While the onions cook, measure out all the spice mix ingredients together in a small bowl.
Add the ginger and garlic and stir fry for 2 minutes till fragrant.
Add the spice mix and fry for 1-2 minutes more.
Add the 1/2 cup yogurt (or tomato if making dairy free), 1 tbsp tahini and boiled potatoes. It will be quite dry, but very fragrant.
Add the 1 can coconut milk and let this come to a boil.
Add the shrimp and let this boil till the shrimp start to turn pink.
Depending on how much liquid you like, you can add more coconut milk or water or allow this to cook for a few more minutes to evaporate some of the liquid. I don't recommend cooking for too long though, otherwise your shrimp will over cook and get hard.
Add the 2 tsp dried methi and stir to combine.
Taste for salt & spice and adjust to taste.