Combine the onion, garlic, oil, salt, pepper, and brown sugar in a large saucepan over medium-high heat.
Cook, stirring often, for 5 minutes or until the liquid has evaporated.
Stir in the treacle, then add the vinegar and oregano.
Continue to cook for another 3 minutes or until the mixture has a thick consistency. Turn off the heat.
Wash the salmon fillets and pat them dry.
Place them in the skillet and sear the fish, about 3-4 minutes per each side.
Pour the sauce over the fish. Add the lemon wedges and cover the pan.
Cook for 25 minutes. Uncover and continue to cook until the salmon is cooked through (the fillets should be opaque).
Serving Suggestions: Serve with baked new potatoes, steamed rice and your choice of green vegetables.