Scale and fillet the salmon trout. Remove the skin and any bones from both fillets and cut across each fillet into two equal portions.
Cut the cucumber across into four 2.5 cm/1 inch lengths, then cut each piece into six and ‘turn’ each section into small barrel shapes, leaving the skin on. Alternatively, cut each piece into six thick batons. Scrape the girolles clean of any dirt or grit, but do not wash. To prepare dried girolles, soak in cold water for about 20 minutes before using.
Butter an ovenproof pan large enough to take the fillets without overlapping and put in the fillets. Add the vermouth and fish stock and cover with buttered paper or foil. Set the pan over a high heat until the liquid just starts to tremble, then transfer the pan to the oven and cook for 4-5 minutes. When cooked, remove the fillets from the pan, cover and keep warm.
To make the sauce, return the stock to a high heat and reduce until syrupy. Add the girolles and the cucumber along with the cream. Bring back to the boil, then remove from the heat and gradually add the butter, shaking the pan with a swirling motion until it has all melted. Do not allow the sauce to boil once the butter has been added. If you need to keep it warm for a few minutes, then stand it in a saucepan of warm water over a low heat.