Mise En Place: Get all of your ingredients prepped and ready. Boil (or microwave--if you're down with that) your potato and drain the liquid from your canned fish. Gather your spices, if you're using them.
Heat oil in a large fry/saute pan over medium-high heat. Tip in the nigella seeds (if using).
Once the seeds begin to dance about in the pan a little bit and your oil is hot, add the turmeric powder and garam masala powder. Let the spices cook in the oil for a minute so that they can release their flavor into the dish and no longer smell raw.
Add the diced onions. Let the onions cook a couple minutes, until they begin to look a bit translucent but not browned. Add the garlic and ginger and cook an additional minute.
Once the garlic and ginger are cooked an no longer smell raw, add the flour. Mix well and cook for a minute or so before adding the boiled mashed potato (try to get that mash as smooth as you can. If you have large chunks, your patties will be more likely to fall apart).
Stir well while cooking so that the potato and the spice mix combine. Then add in the drained canned tuna fish (or cooked, flaked fish), frozen peas and minced coriander leaves (cilantro). Stir well to combine. Season with a squeeze of lemon, salt and pepper to taste.
Remove this mixture into a bowl and let cool for a few minutes before adding the egg white (or whole egg + additional tbsp flour). Mix well and then scoop into even balls (I use a large cookie scoop). Flatten the balls into patties.
Using the same pan you used to cook the mixture, heat another tablespoon of oil over medium-high heat. Once the oil is hot, add the patties--being sure not to crowd the pan. Let the fishcake cook for a few minutes or until it begins to lightly brown, then carefully flip the fishcake--taking care not to break it. Cook the other side for a few more minutes until it also begins to brown. (I cooked the cakes for 7-8 minutes total).
Remove the fishcakes from the pan onto a paper-towel lined plate. We like to sprinkle ours with a bit of chaat masala (an indian spice blend) or some himalayan black salt to add an extra kick of "sourness." If you don't have access to these ingredients--you can always just squeeze a bit of lemon or lime, to taste.