Preheat oven to 200 c (400 f /Gas 6). Cut the potatoes into even pieces, put into a pan of water and boil until just tender.
Drain the potatoes, return to pan ad dry over a low heat for a few moments. Add the butter and egg yolk and mash together. Stir in the chives and tuna.
Divide the mixture into 6 equal portions and , with floured hands, shape into flat pear shapes. Shape the thinner end of the cakes to form a 'V' shape like the tail of a fish.
Put the beaten egg into a shallow dish. Place the fish cakes in the egg and brush with egg.
Coat in bread crumbs, then place on a greased baking sheet
Brush with a little oil and bake for 20-25 minutes until crisp and golden.
To serve, place a pea on each fish for and "eye" and add a small sliver of tomato for the mouth. Serve hot.