Begin by making your curry paste. In a non-reactive mixing bowl, combine first six ingredients for the curry. Give the ingredients a good stir after adding the lemon and before adding the coconut milk. Then, gently stir again until the paste has the consistency of a creamy yogurt. Fold in the cilantro.
Heat oil in a ceramic or stainless steel braising pan, and add your curry leaves; fry until fragrant.
Add black mustard seeds and garlic; continue to fry until garlic begins to brown.
Add shrimp, all at once, to the heat; the shrimp may sputter oil. Take care to ensure that shrimp can cook evenly. If the pan is hot enough, turn the shrimp after about 1-2 minutes cooking time.
Add the curry paste, cover, and keep stove on medium-high heat for one more minute.
Remove from heat and/or allow to simmer gently, being mindful not to overcook the shrimp.