Steep the kudampuli in enough hot water to cover. In a separate bowl, mix the masala (coriander, Kashmiri chilli, turmeric, and black pepper) and set aside.
Heat oil over a medium flame until it shimmers before adding the mustard and fennel seeds. When the mustard begins to splutter, add the fenugreek. Fry until they darken—this will only take a few seconds. Then, add half the curry leaves. When they frizzle and darken add the onion and cook until they soften and turn colour. Then, add the ginger, garlic, and slit chillies.
Add the masala and fry until the spices exude their oils and release their scent.
Add the kudampuli water and thin coconut milk. Bring the mixture to a boil. Lower the flame to medium-low and let the mixture die down to a simmer and stir for about 8 minutes as it thickens.
Add the shrimp, tomatoes, and salt. Stir for about 5 minutes, or until the shrimp is opaque and just cooked. Stir in the thick coconut milk and the remaining curry leaves. Remove from heat when it comes to a boil. If the tomates are still firm, lid the pan and let them steam until soft. Taste for salt and serve.