Make a bag for your fish by tearing off a 35cm x 45cm strip of tin foil and folding it in half across the middle. Fold three of the sides up like an envelope, sealing the edges by brushing them with a little beaten egg before folding. Leave one side open.
Roughly chop or slice the potatoes, then cook in a pan of boiling salted water for 6 minutes. Drain and leave to cool slightly.
Trim and chop the fennel into wedges, reserving the fronds, then finely slice the lemon. Halve the cherry tomatoes, then squash the olives and remove the stones.
Place the fennel, lemon, tomatoes and olives into a large bowl with the cooled potatoes and fish. Drizzle lightly with oil, and season with sea salt and black pepper, then toss gently to coat.
Carefully transfer everything to the foil bag, sprinkle in the fennel fronds, then loosely fold up the open side and pop the bag in the fridge until needed.
When you’re ready to cook, preheat the oven to 200ºC/400ºF/gas 6.
Unravel the loose side of the bag, pour in the wine, then reseal and place on a baking tray.
Cook for 18 to 20 minutes, or until the fish and potatoes are cooked through.
To serve, place the bag on a serving plate and gently pierce to release the steam – the tomatoes and wine will have made a delicious sauce. Delicious served with steamed broccoli or green beans.