Gratin de cardons

Step 1: Fill a large bowl with cold water and squeeze in 1 lemon, dropping the halves in too.

Step 2: Discard the tough outer stalks and leaves of 3 ⅓ lb cardoons. Use a vegetable peeler to strip the stringy ribs off each stalk, cut into 6 cm lengths and drop straight into the lemon water.

Step 3: Whisk 2 tbsp plain flour into 3 litres of cold water in a large pan with 2 tsp salt, bring to the boil and add the drained cardoons. Simmer for 35–45 minutes, until a knife slides through with no resistance. The flour keeps them pale.

Step 4: Meanwhile, melt 1 ⅜ oz butter in a pan, stir in 1 ⅜ oz plain flour and cook for 2 minutes over a medium heat without colouring. Gradually whisk in 1 ¾ cups whole milk and ¾ cup vegetable stock and simmer for 5 minutes, stirring, until it coats the back of a spoon. Season with ¼ tsp ground nutmeg, ¼ tsp white pepper and salt.

Step 5: Heat the oven to 400°F (350°F fan). Drain the cardoons very well and pat dry. Common mistake: putting them in the dish still wet, which waters down the sauce into a thin puddle.

Step 6: Spread the cardoons in a buttered gratin dish, pour over the sauce and scatter with 2 ½ oz Gruyère.

Step 7: Bake for 25 minutes, until golden and bubbling at the edges. Rest for 5 minutes before serving. You can assemble it a day ahead and bake from cold for 35 minutes.