Cut the cabbage head into narrow strips, boil with a little salt. Cook until crisp, drain and squeeze out all the remaining water. Drain the tuna and cut into pieces. Mix all ingredients, add as much breadcrumbs until a doughy consistency is reached, season to taste.
Spread the mixture in an oiled flat ovenproof dish and garnish with onion rings. Bake in oven at 180 °C for about 40 minutes. Serve cold or warm with hot sauce.