Put the potatoes, together with one of the sprigs of thyme and one of the bay leaves, in a large saucepan filled with about 1.5 litres (6½ US cups) of lightly salted water. Bring the water to the boil, adjust the heat to maintain a brisk simmer and cook the potatoes for about 15 minutes or until just cooked. Strain; discard the herbs and set aside the potatoes in a warm place.

Using a non-stick frying pan (skillet), sauté the mushrooms lightly in about 1 teaspoon of the olive oil; set aside.

Simmer the onions in lightly salted water for about 10 minutes or until just tender; drain and set aside in a warm place.

Bring the chicken stock to the boil along with the lemon juice and the remaining sprig of thyme and the bay leaf. Remove it from the heat, discard the herbs and gradually stir in the cornflour (cornstarch) mixed with water. Adjust the seasoning and keep this stock warm, preferably in a bain-marie.

In a non-stick frying pan (skillet), heat another teaspoon of the oil and colour the pieces of brill on one side only; season.

Preheat the oven to 180°C (355°F, gas mark 4). Spread out the circles of parchment paper on a flat work surface and brush them with the remaining teaspoon of the olive oil. Leaving half of each circle free, put a little bed of mushrooms towards the centre of the other half. Put a piece of the brill on top of each, with its coloured side uppermost.

Distribute the potatoes and onions around the brill. Divide 2 tablespoons of the reserved chicken stock between each parchment circle. To close each parcel and make a leakproof seal, bring together the edges of the paper and fold them over twice to make a narrow seam all along the edge, like an apple turnover.

Transfer these little parcels – papillotes – to a lipped baking sheet and put the sheet in the oven. Cook for 8–10 minutes.

To serve

Have ready 4 warm serving plates. Remove the baking sheet from the oven and transfer the papillotes to the plates. Divide the wedges of lemon between the plates. Serve the papillotes closed so that guests can have the surprise of opening them at the table. Serve the remaining flavoured stock in a sauce boat and offer it separately.

Variations

You can replace the brill with halibut, salmon or striped bass if you prefer.