Heat the oven to 220C. Heat a large baking sheet in the oven too.
Cut out 2 large heart shaped pieces of good quality baking parchment.
Put mackerel fillets flesh to flesh, on one side of the paper. Sprinkle with lemon grass, ginger and red chilli slices. Fold the paper to seal in the mackerel, but pinching the edges together.
Put the wrapped paper-cases on the heated baking sheet and bake in the oven for 8 minutes.
In the meantime mix the dressing ingredients together. Arrange to have some noodles to hand for serving.
After 8 minutes lift the paper cases from the oven and tear them open. Spoon the hot and sour sauce over the mackerel in their cases and sprinkle with sprigs of coriander. Lift the paper cases onto a serving dish and hand the noodles separately and garnish with coriander and lime wedges