Pat dry olives, blend all ingredients together.
The breadcrumb mix should be mostly dry after pouring out of the blender.
Pat dry fish. Brush a thin layer of mustard on top and scatter breadcrumb mix on top. Pat lightly to form a layer of olive crust.
Place fillet in oven at 210°C for 8 minutes.
Combine diced onions, citrus juice, cooking wine in saucepan. Cook on medium heat. Boil until shallots have turned translucent and some liquid has evaporated. Add slurry into saucepan and turn off heat once it thickens. Add cold butter and shake the pan as it melts into hot liquid. Season with salt and pepper.
Insert metal skewer or metal chopstick into centre of the fillet and leave it for 5 seconds. Pull out and feel for the temperature. It should be nicely warm for a fish fillet to be perfectly cooked. Leave it in the oven to cook longer if the skewer feels cold.
Finish plating by spooning sauce onto the bottom of the plate, and lay the crusted snapper on the sauce.