Preheat oven to 350º F.
Combine mirin and sake in a small saucepan and bring to a boil, allowing alcohol to burn off. (Always be careful when cooking with alcohol, as it can cause a small flame.)
Turn the heat to low and add red miso paste and sugar, whisking to combine. Continue cooking over low heat, stirring occasionally, for about 15 minutes (at just a very slight simmer with only occasional bubbles) to allow the glaze to thicken and reduce.
Remove from heat and set aside.
Meanwhile, in a small bowl, combine green onions, ginger, oil, tamari, vinegar, 1/16 teaspoon salt and 1/32 teaspoon pepper. Set aside (or place in the refrigerator until serving).
Meanwhile, season the flesh side of the salmon evenly with 1/4 teaspoon salt and 1/16 teaspoon pepper.
Preheat an oven-proof skillet over high heat. Once your skillet is pre-heated, add 1 teaspoon oil, quickly swirling to coat, and then immediately add the salmon, flesh side down. Sear the salmon for 1 minute on high heat, and then turn the heat down to low and continue cooking for 3 minutes more. (The salmon should develop a lovely golden-brown crust and release easily from the pan when you try to flip it.)
Remove the salmon from the heat. Carefully turn the salmon over with a large spatula, so the seared, flesh side is up. Using a spoon or pastry brush, spread 2 tablespoons of Red-Miso Glaze over top of the salmon.
Place the skillet of glazed salmon into the preheated oven for about 5 minutes for medium-rare. Serve immediately.
Serve salmon drizzled with additional Red-Miso Glaze and topped with Ginger-Scallion Sauce. We also like to drizzle a little additional glaze over the rice or veggies we serve alongside.
Make ahead steps: Both the Red-Miso Glaze and the Ginger-Scallion Sauce can be made up to a day ahead, and stored separately, covered, in the refrigerator. If you'd like to thin the Red-Miso Glaze slightly prior to using, you can microwave it gently for 10-20 seconds to take the chill off and make it slightly less thick.