Peel the potatoes and cut them into chunks. Cook them in boiling salted water until they are soft. This will probably take about 20 minutes
Put the stock into a saucepan and then add the chopped fish. Cook in the stock for 5-10 minutes
While the fish is cooking, sauté the spring onions until they start to soften
Place the cooked spring onions and peas into your pie dish and then remove the fish from the stock using a slotted spoon. Add the fish to the pie dish along with the prawns and mussels
Return the fish stock to the heat and bring up to the boil. Add the chopped parsley and a squeeze of lemon juice, then whisk in the xanthan gum until it thickens. Add a little at a time and whisk it really well so that it dissolves and there are no lumps
Remove from the heat, stir in the quark and then pour it over the fish mix
Drain the potatoes and mash them until they're smooth. Add a little salt and pepper to taste, then fill a piping bag and neatly pipe the potato on to the fish mix
Cook at 200°C (fan) for about 30-35 minutes or until the top is golden brown