Preheat the oven to 200C. Score both sides of the flesh of the skin.
Insert into the belly the lemon, grapefruit, garlic and fennel into the belly of the fish. Drizzle the skin with olive oil and salt.Bake at 200C for 5 minutes, then for a further 10min at 180C.
Lightly drizzle the Brussel sprouts with olive oil and salt. Bake with the trout for 10min until bright green and crispy.
Once the Brussel sprouts are cooked, assemble the salad by combining the spinach, apple slices and sprouts. Make the dressing by emulsifying the oil, lemon and grapefruit juice, mustard and salt. Drizzle over the salad and scatter nuts.