If making ahead, brush the flesh side of the salmon fillets with a layer of the miso-orange glaze, cover, and refrigerate overnight (optional).
Make the glaze: combine mirin, sake, sugar, and orange juice in a small saucepan over low heat; whisk in the miso paste, then bring to a gentle boil for about 1 minute while stirring. Remove from heat; the glaze will thicken as it cools.
Preheat the oven to 180°C (350°F). If you haven't already glazed the salmon, brush a layer of glaze onto the flesh side now.
Heat a non-stick frying pan or ovenproof skillet over high heat. Rub a little olive oil onto the salmon skin and season the fillets with salt and pepper.
Sear the fillets skin-side down for 3–4 minutes, until the skin is golden and crispy.
Brush another generous layer of glaze over the flesh of each fillet, then transfer the pan to the preheated oven (or move fillets to a parchment-lined baking sheet) with the flesh side facing up.
Roast for about 12–15 minutes, brushing with more glaze about halfway through, until the salmon is cooked through. Reserve any remaining glaze to serve.
Remove from the oven and serve warm with the reserved glaze.