1. Preheat oven to 180°C (160°C fan-forced) and line a large baking tray with baking paper. Place trout on baking tray and season inside the cavity with sea salt and freshly ground black pepper. Stuff the cavity with the citrus slices, overlapping slightly, then stuff with some of the reserved fennel fronds. Tie the fish at regular intervals with kitchen string to help hold the stuffing in place.
2. Drizzle trout with extra-virgin olive oil, season with sea salt and freshly ground black pepper and roast until the thickest part of the fish flakes easily (45-50 minutes). Remove from oven and set aside covered loosely with foil to rest while you make the sauce.
3. Cook the butter in a saucepan over medium-high heat, swirling the pan occasionally, until golden brown and fragrant (4-5 minutes). Remove from heat, carefully add the citrus segments (be careful as the hot butter may spit), then add pistachios and garlic and season generously to taste.
4. To serve, spoon a little of the brown butter sauce over the trout and scatter with fennel fronds. Serve with potato and fennel gratin and David Jones Cabbage & Kale Slaw.