In a small pot, add mirin, sake, sugar, and miso paste. Whisk until smooth.
Place the pot over medium heat and gently bring the mixture to a simmer, about 3–4 minutes, stirring occasionally, just until the sugar dissolves and the alcohol cooks off. Remove from heat.
Once the marinade has completely cooled, place the salmon in a glass storage dish and pour the marinade over it. Lay a piece of parchment paper directly on top so the salmon stays fully submerged.
Cover the dish and marinate in the refrigerator overnight, or up to 3 days.
Air Fryer Instructions: When ready to cook, remove the salmon from the marinade and let excess drip off. Air fry 3 filets at 400°F for 12 minutes.
Oven Instructions: When ready to cook, remove the salmon from the marinade and let excess drip off. Place the fillets on a parchment-lined baking sheet. Bake at 425°F (220°C) for 10–12 minutes, or until the salmon is just cooked through and flakes easily with a fork. Optional: Broil for the last 1–2 minutes for a lightly caramelized top.
