For the ketchup:
Boil in a pan the pureed tomatoes with half the vinegar, brown sugar, soy sauce and a pinch of salt for 10 minutes over a low heat.
Blend the mozzarella into a cream.
Slice the onions thinly and caramelise in a frying pan with 1 tablespoon of brown sugar, remaining vinegar, two pinches of salt, and the bay leaf for 15 minutes over a low heat.
For the tuna burger:
To assemble the burgers, spread the cut buns with the mozzarella cream and the homemade ketchup, add one lettuce leaf, the tuna burgers, then the onions, mustard and finish off with more ketchup and another lettuce leaf.
Mince the tuna, using sharp knives or a food processor, add the spices and form into burgers; grill for 3 minutes.