Preheat the oven to 200°C (180°C fan)/Gas 6 and line a baking tray with baking parchment or foil.
In a small bowl, whisk together the miso paste, mirin, reduced-salt soy sauce, caster sugar, finely grated fresh ginger and crushed garlic until smooth.
Pat the salmon fillets dry with kitchen paper and arrange them on the prepared tray.
Spoon the miso mixture over each fillet to coat. If you have time, leave at room temperature for 15-20 minutes to marinate.
Bake for 12-15 minutes, or until the salmon is cooked through and flakes easily with a fork.
Switch the oven to the grill setting and grill for 1-2 minutes, watching closely, until the glaze is golden and bubbling.
Remove from the oven, drizzle over the sesame oil if using, and scatter the spring onions on top before serving.