Place the fish, skin-side up, on an oven tray and place a lightly oiled cake rack on top of the fish. Using a domestic blowtorch, sear the skin for 10 seconds or until the skin has been just marked. Remove the cake rack and refrigerate the fish until needed. For the achara, place the chilli vinegar and sugar in a saucepan over low heat. Stir until the sugar dissolves, then increase heat to how and bring to the boil. Remove from heat. Cool, then add the garlic. Add the remaining ingredients, combine well and stand for at least 10 minutes. Drain and refrigerate until ready to serve.
Combine the sea grapes, tomato and onion in a bowl. Set aside.
To make the vinaigrette, place all the ingredients in a bowl, season to taste and combine well.
To serve, thinly slice the fish and arrange on plates. Top with the sea grape salad and serve the achara separately with soy sauce.
Notes: Calamansi are a citrus fruit similar to lemons and are widely used in the Philippines. You can substitute with lemons. Sea grapes are a type of seaweed that forms in clumps that look like a bunch of grapes.