Place all of the marinade ingredients in a resealable plastic bag. Add the salmon and leave to marinate for 2 hours in the fridge.
Once salmon is ready to cook, remove salmon from the bag and discard the marinade. Heat 1/2 the olive oil in a frying pan. Pan fry the salmon fillets for 1 to 2 minutes on each side, taking care not to overcook the fish. Set aside.
Heat the remaining olive oil in the same pan and saute the onion till translucent. Add the oyster sauce, sugar, pepper and water and simmer for 2 minutes.
Pour the sauce onto a serving platter, and place the salmon on top of the sauce. Garnish with thyme sprigs to serve.