Place the mackerel in a long gratin dish with the lemon grass, garlic and chilli. Add the oil, lime juice and rind and seasoning, mix well and then add in the fish. Turn once to cover well and then leave for up to one hour.
Remove the fish from the marinade and grill or barbecue fast for 2 minutes each side.
Place the marinade in a small pan with the Pastis and cook only until the liquid bubbles and the gooseberries begin to fall. Then serve a little with each fish.