Grill the kippers for a few minutes on each side, then set them aside to cool.
Meanwhile make the pastry and line a 9-91/2 inch (23-24cm) fluted flan tin with it. Prick it all over the base with a fork and bake on the baking sheet near the top of the oven for 10 minutes, then remove the flan and increase the temperature of the oven to gas mark 5/375F/190C. Next carefully skin the fish, separate the flesh into large flakes, lay these over the base of the pastry case and squeeze a little lemon juice over the top.
Now whisk the milk, eggs, mustard and freshly grated nutmeg together with some salt and pepper. Place the flan on the heated baking sheet in the centre of the oven, pour the liquid mixture carefully into it and sprinkle with cayenne pepper. Then bake for about 40 minutes, until the filling is slightly puffed and golden. Served hot, warm or cold, it is delicious.