Preheat oven to 230°C/210°C fan-forced. Line a baking tray with baking paper. Place potato on tray, drizzle with oil and season. Bake for 12 minutes.
Meanwhile, combine miso, mirin, soy, ginger and sugar in a bowl. Reserve half the mixture and set aside. Add salmon to remaining mixture, turning to coat.
Move potato to one side of tray and top with broccolini. Add salmon to other half of tray and bake for a further 12 minutes, spooning reserved miso mixture over salmon in last 5 minutes of cooking or until potato and broccolini are tender. Move vegetables back over entire tray, then arrange salmon on top. Serve scattered with sesame seeds, chilli and onion.