Pull legs and claws off crabs, then use a nutcracker to crack them at the joints. Discard any bits of shell that come flying off. Pile the legs, claws and body sections on a plate.

Pulse peppercorns in a mini food processor until coarsely ground. Transfer to a small bowl. Add garlic, ginger, turmeric and 2 tablespoons water to the food processor and process until a coarse paste forms. Return the crushed pepper and pulse to combine. (Alternatively, pound the aromatics with a mortar and pestle, adding the water gradually.)

Stir the remaining 3/4 cup water, oyster sauce, fish sauce and sugar in a small bowl to combine. Set near the stove, along with the crabs and the seasoning paste.

Warm oil in a large flat-bottom wok or large pot over medium-low heat. When hot enough to gently sizzle a bit of the seasoning paste, scrape in all the paste. Cook, stirring frequently, until fragrant and the oil starts separating from the paste, 3 to 5 minutes.

Add the sauce mixture to the pan, then increase the heat to high. When the mixture begins to bubble, dump in the crab. Stir-fry until the crab is heated through and coated evenly with the seasoning paste, 3 to 5 minutes. Remove from heat and stir in cilantro, if using. Arrange the crab on a large platter and serve immediately.