Heat oil in a large, deep frying pan over medium high heat. Toss chicken in cornflour and place in frying pan with garlic, onion, salt and pepper. Stir, lightly browning chicken all over and softening onions. Add sherry and beef stock and let liquid reduce a little.
Stir in peanut butter, curry powder and ginger and enough water to cover. Reduce heat to low and simmer for 30 minutes. Just before serving, stir in coconut milk, heat through and serve hot.