For the smoked salmon butter, place the butter, smoked salmon and lemon juice in a food processor and blitz until smooth. Season and set aside.
Trim, halve and slice the leeks lengthways, then add the butter, oil, bay leaves and thyme to a wide, shallow saucepan, place over a medium-low heat and bring to a simmer. Add the leeks and cook for 10 minutes, turning, until golden.
Pour in the stock and wine, cover with a large circle of baking paper and cook for 15 minutes or until leeks are tender. Remove the pan from the heat and rest the leeks in the liquid.
Put the capers in a small saucepan and add enough olive oil to cover them by 2cm. Place it over a high heat and fry until the capers begin to open and crisp up. Using a slotted spoon, transfer the capers to a plate lined with kitchen paper.
Heat a griddle pan over medium–high heat. Brush the ciabatta with olive oil and toast until golden, about 1 minute each side.
To serve, spread the toasted ciabatta generously with the smoked salmon butter and top with the leeks, fried capers and pick over a few chervil leaves.