To prepare the sauce, combine shoyu, rice vinegar, oyster sauce, brown sugar, garlic, ginger, and lemongrass. Mix and set aside.
Line steamer with parchment paper, creating a pouch shape. Place salmon filet inside. Pour sauce on top and gently rub the sauce all over the salmon.
In a wok or skillet, cover the bottom with water, but not to the point the water will touch the bottom compartment of the steamer. Heat up to a gentle boil, or when steam starts appearing. Place steamer directly on the water covered and allow steam for 10-12 minutes. Don't remove the lid. Once ready, set steamer aside and drain the water.
In the same skillet, sauté mushrooms, onion, and snap peas just enough to kiss the fire. Pour in sauce from the salmon and let reduce to a thicker consistency. Immediately remove the heat. Serve vegetable stir fry on top of the Asian style steamed salmon and enjoy with a side of rice!