This effortless technique of cooking in a paper parcel gently steams fish for a delightfully tender result. A miso-enriched butter, earthy shiitake mushrooms and crispy sweet potato dial things up without taking over the delicate flavour of the fish. If you have someone to impress, you could call it 'en papillote', which is the French term for cooking in paper. So chic.
Preheat oven to 200°C fan-forced. Remove butter from fridge and allow to soften. Peel and cut sweet potato into wedges and place on a large baking tray lined with baking paper. Drizzle with olive oil and season with salt. Roast for 20 minutes or until just tender (you will need to take the tray out halfway through cooking to add the fish parcels).
Trim ends from bok choy and cut into 4cm pieces. Trim ends from spring onion and cut in half. Cut halves into matchsticks.
Place miso paste, mirin and butter in a small bowl and mix until well combined.
Cut 2 x 30cm-long rectangles of baking paper and place on a clean benchtop. Place a fish fillet on each rectangle. Divide shiitake mushrooms, bok choy and ½ the spring onion between parcels. Divide miso butter between parcels and drizzle each parcel with 2 teaspoons water.
Bring 2 sides of the paper to the centre and fold over twice to seal. Fold ends enclose and tuck underneath the parcel. Remove sweet potato tray from oven and make space for fish parcels. Add fish parcels to the tray and bake for 12–15 minutes or until fish is just cooked through and sweet potato is crispy.
Remove fish and sweet potato from oven and set aside for 2 minutes to allow fish to continue to steam. Transfer parcels to plates and top with remaining spring onion. Serve with sweet potato wedges.