In a large saucepan, combine the vinegar, water, onions, bay leaves, curry powder, sriracha wasabi sauce, peppercorns, and sugar. Bring to a gentle boil while stirring consistently.
Cook for about 15 minutes, adjusting the curry to taste if more heat is needed.
In a small bowl, mix the cornflour with a little of the curry sauce to create a smooth paste.
Slowly stir the paste back into the saucepan, continuing to stir until the sauce thickens slightly.
In a prepared oven dish, pour the curry sauce and arrange the fish medallions in the sauce.
Bake uncovered for 30 minutes.
Transfer to a container and store in the fridge.