If your prawns or shrimp are not shelled and clean, begin with that task. Place them in a bowl and coat them with the salt and turmeric. Set aside until needed.
Toast Whole Spices
Gather the rest of your ingredients.
In a small pan dry toast the whole spices. Remove the seeds from the dry red chilies if you want to reduce their heat. Toast the spices for several minutes until they are fragrant.
Grind the spices in a spice grinder. Mix together the ground spices, vinegar, ginger, and garlic. Set aside.
Make the Sauce
In a large pan over medium-heat, when hot, add the oil and the onions. Sauté them until they turn golden brown, about 6 to 8 minutes. Add the tomatoes and cook until the moisture evaporates, the sauce thickens, and the oil begins to separate. This will take another 5 minutes or so.
Add the spice and vinegar mixture to the tomato and onions and cook for about 3 minutes, or until the sauce no longer smells raw. Add the shellfish and sugar. Cook gently for a couple of minutes or until the shrimp are just cooked through.
Taste for seasonings, adding sugar, salt, or a bit more acid if needed. Make sure that you add enough salt. Heavily spiced dishes need more salt than you may think. If the sauce is too intense, add some yogurt or cream to mellow it out.
Preparation shortcut: buy shrimp that has already been cleaned and is ready to cook.
I use whole Kashmiri red chilies. If you want a very hot curry, use hotter chilies and add more to your taste.
I don’t prefer plum tomatoes because they are firm and take twice as long to cook down.
I made this curry a rather dry dish. If you want more sauce, double all the ingredients except the marinade: shrimp, 1/2 teaspoon salt, and turmeric. I suggest this so the flavors stay in balance.