For the salad plate with tuna, wash the iceberg lettuce, peppers, cucumber and tomatoes and cut into small bite-sized pieces. Arrange everything on a large deep plate.
Slightly pull apart the tuna and spread it over the salad. Peel the onion and cut it into fine rings and spread it on the salad as well.
Mix the tomato ketchup with oil, water and the peas and season to taste. Pour the dressing over the salad.