Wholemeal sandwich with tuna salad

A tasty Dutch recipe. The snack contains the following ingredients: fish, tuna ((185 g), drained), red onion (very finely chopped), freshly sour apple ((eg Granny Smith), in small pieces), Amsterdam onions ((jar 340 g) , halved), mayonnaise (light-wood, jar 450 ml) and wholemeal bread with sesame (toasted).