Combine the tamarind sauce ingredients in a mixing bowl with a pinch of sea salt. Mix well, and then set aside.
Add the vegetable oil, garlic, shallots and onion to a hot wok. Stir-fry over a high heat for 30 seconds, or until fragrant.
Add the peppercorns and crab and toss for 7 minutes. Add the tamarind sauce and cook for a further minute, or until the crab pieces are cooked through.
Add the basil and chilli, toss once and serve. Garnish with more basil and chilli, if desired.