In a large frying pan, heat the oil over high heat. Add the salmon in 2 batches, and cook each batch until the fillets are lightly browned, about 3 minutes. Turn them over and cook their second sides for 1 minute, then remove them from the pan.
Reduce the heat to medium, and add a little extra oil to the pan, if needed. Add the onion and garlic and cook for 3 minutes, stirring occasionally. Stir in the soy sauce, sugar and fish sauce. Return the salmon to the pan and continue cooking until the sauce is rich, dark and syrupy, about 1 minute. Season the dish liberally with black pepper, and stir in the lime juice. Serve the salmon alongside some steamed rice, and drizzle the dish with the sauce, and garnish with lime wedges.