Cook the egg in boiling water for about 10 minutes. Pour off, rinse with cold water and peel, allow to cool. Drain tuna and capers. Peel onion and cut into rings. Roast the toast bread in a toaster until golden brown.
Coat with grease. Cut the egg into slices. Cut up tuna fish. Cover toast with tuna, egg, capers and onion rings. Sprinkle with cheese. Place toast on a baking tray lined with baking paper.
Bake in the preheated oven (electric oven: 200 °C/ convection oven: 175 °C/ gas: level 3) for 5-8 minutes until the cheese has melted. Garnish with dill.