Fry the garlic and shallots together until browned. Set to one side to cool.
In a food processor, mix the fish until it becomes almost like a paste. Add the cooled onions, breadcrumbs, egg, rosemary, chilli, sea salt and pepper. Mix until everything is nicely combined.
Taking a burger mould or any round cutter, fill it with the fish mixture (make them as small or big as you like) and press down. Place on a lined baking tray and pop in the fridge until you are ready for them.
Pre-heat the oven to 200ºC (400ºF). Using a tablespoon or so of olive oil, gently fry the burgers for around 5 minutes on each side. Remove from the pan and place back on the oven tray and cook for a further 5-10 minutes.
In the meantime, make the mayo: add the finely chopped shallot to a bowl with the mayo, vinegar and chilli powder. Mix thoroughly.
When ready to assemble, cut the bread bun open. Slather over some spicy mayo, add a few salad leaves and place the burger on top. Serve immediately. Enjoy!