1. Line a flan case with the pastry and bake blind for about 10 minutes.
2. Fry the onion in the butter and add the laver, mussels, milk, cream, beaten eggs and seasoning. Fill the flan case, sprinkle with parsley and garnish with the sliced tomatoes.
3. Bake in the oven on Gas Mark 7, or equivalent, for 10 minutes, then on Gas Mark 4, or equivalent, for 20 minutes.