In a large skillet, heat the oil and sauté the salmon for a few minutes on each side. Discard the oil in the pan and add the butter. As soon as the salmon is coated with melted butter, remove the fish from the pan with a spatula to individual plates.
To the butter in the pan, add the rice vinegar, mirin and shoyu. Stir over high heat for about 1 minute. Add the green onion and stir briefly. Drizzle the sauce over the salmon steaks, garnish with lemon wedges and serve.