2) While the barbeque is heating, lay the salmon skin-side down on a cutting board and cut it crosswise into 4 equal pieces. Whisk together the olive oil, mustard, soy sauce and garlic in a small bowl. Drizzle half the marinade onto the salmon and allow it to sit for 10 mins.
3) Place the salmon skin side down onto the hot barbeque; discard the marinade the fish was sitting in. Cook for 4 to 5 mins, depending on the thickness of the fish.
4) Turn carefully with a wide spatula and cook for another 4 to 5 mins. The salmon will be slightly raw in the centre, but don't worry; it will keep cooking as it sits.
5) Transfer the fish to a flat plate, skin side down, and spoon the reserved marinade on top. Allow the fish to rest for 10 mins. Remove the skin and serve warm, at room temperature, or chilled.