Prepare the Oven: Adjust an oven rack to sit about 6 inches from the top heating element. Turn the broiler on HIGH to preheat for at least 5 minutes.
Mix the Glaze: In a small bowl, whisk together the low-sodium miso, rice vinegar, coconut aminos, maple syrup, toasted sesame oil, minced garlic, grated ginger, and water until completely smooth.
Glaze the Salmon: Pat the salmon fillets completely dry with a paper towel and place them in a 10-inch oven-safe skillet, skin-side down if the skin is on, in a single layer. Spoon about two-thirds of the glaze over the tops of the fillets, spreading it evenly.
Broil: Place the skillet under the hot broiler for 4 to 6 minutes, watching closely so the glaze does not burn. This initial cook time will cook the salmon most of the way through.
Finish with Final Glaze: Carefully remove the skillet from the oven. Brush the remaining glaze over the salmon. Return it to the broiler for another 1 to 2 minutes, just until the glaze is bubbling and the fish is cooked through. The salmon is done when it flakes easily with a fork or an instant-read thermometer inserted into the thickest part registers 145°F / 63°C.
Garnish and Serve: Let the salmon rest for a minute. Garnish with toasted sesame seeds and sliced green onions before serving immediately.