Lightly hit the lemongrass stalks with a rolling pin to release the flavour - great fun for the kids! Peel and thinly slice the ginger. Remove the seeds from the chillies and chop finely. Wash the pak choi and separate out the leaves, again getting the kids to help while you trim and chop the shallots. In a large bowl, combine the lemongrass stalks with the lime leaves, chilli, peppercorns, ginger, coriander seeds and leaves. This is a good chance to teach the little ones that lime leaves are used in a lot of Southeast Asian cuisine and can be used dried or frozen. Add the pak choi, sesame oil, chopped shallots, rice wine vinegar and 4tbsp water to the bowl and mix together well. This is your marinade that you will use to flavour the fish. Cut 2 squares of foil 30cm (12in) wide. Then cut 2 pieces of nonstick baking paper to the same size. Place 1 square of foil on top of each piece of nonstick baking paper and fold them both in half, then open out again so that they have a crease down the middle. Spread some butter across the middle of the opened parcels. Lay a haddock fillet over the crease in the middle of each one. Place 1 of the foil wraps with fish in a bowl, so the contents will stay contained and the marinade won't run out when you pour it in. Pour half the marinade over the fillet, then crimp the edges together to make a sealed bag with the fish and liquid inside. Repeat the process with the second bag of haddock fillet. Place the bags on a baking tray, being careful not to lose any liquid, and put in the oven for 10 minutes - the bags will puff right up and the fish will steam. Keep the bags sealed until you are ready to serve. Place the parcels on plates and open the wraps just before serving.