Peel and devein shrimp/prawns, if not already prepared. Place shrimp in a small bowl and sprinkle over the turmeric, salt and lemon juice. Stir to coat evenly then set aside for around 10 minutes to marinate while you prepare other ingredients.
Finley chop the onion, garlic, ginger and chili. Remove skin from the tomato and chop.
When ready to cook, warm around half of the oil in a skillet/frying pan, wok or wide pot/pan over medium heat. Add the shrimp and cook to sear on both sides around 2 - 3 minutes each side. You don't need them fully cooked but they should turn a bright color on either side as the grey turns to pink. Remove from skillet and set aside.
Add the rest of oil then add the curry leaves, fenugreek and mustard seeds. Cook for a couple minutes, stirring occasionally, until the seeds start to splutter.
Add the onion and cook around 5 - 7 minutes so that it becomes well softened. Add a little additional oil if needed so that the onion doesn't burn. Add the garlic, ginger and chili and cook a couple minutes more so that they become fragrant and softened. Stir relatively regularly as they cook.
Add the dried spices - coriander, kashmiri chili and turmeric - and mix them through well. After a minute or two add the tomato, stir to soften and cook briefly, then add the coconut milk. Stir to mix well and scrape any spices that have stuck to the bottom (it's all tasty flavor, though don't scrape if it has burnt which hopefully is not the case. Browned is fine.).
Bring the coconut base to a simmer and add the tamarind, stir through, then add back the shrimp. Cook for a minute or two so that the shrimp are cooked through, but take care not to over cook, then serve eg with rice, paratha or naan.