Grind coconut, chile peppers, coriander seeds, garlic, and peppercorns together using a mortar and pestle until masala is evenly orange.
Heat oil in a pot over medium heat; cook and stir onion until softened and lightly browned, 5 to 10 minutes. Mix masala and turmeric into onion; cook until fragrant, about 1 minute. Add enough water to make a substantial and smooth gravy.
Bring gravy to a boil; add prawns and okra and cook until prawns are cooked through and okra is tender, about 10 minutes. Stir kokum into prawn mixture and return to a boil. Remove pot from heat and let stand.