Peel and finely chop the garlic and ginger.
Rinse the tuna, pat dry well with kitchen paper and cut into cubes. Pulse in a food processor to a coarse mince.
Mix the tuna with the garlic, ginger and mustard. Season with salt and pepper. Divide into 4 burgers and chill in the fridge.
Peel the onions and cut into rings (toss in flour if desired). Heat the rapeseed oil in a pan and fry the onions until golden brown and crisp. Remove from the pan and drain on kitchen paper.
Cook the fish burgers in the onion frying pan for 3-4 minutes on each side until golden brown.
Mix together the mayonnaise, sour cream and sesame oil, then season with salt and pepper.
Halve the bread horizontally and toast. Wash the rocket and spin dry. Wash the tomatoes, remove the stems and dice. Spread the undersides of the toasties with the dip and put on plates. Arrange the rocket around the edge.
Put the meatballs and onions on the rocket. Decorate the remaining toast and sprinkle with diced tomatoes.