If possible, fillet and debone the trout at the fishmonger`s (take fish scraps with you, however.)
Drizzle the fillets with a little lemon juice and set aside.
Rinse off the fish waste and place in a saucepan with water, wine, onion quarters and soup greens. Add parsley, peppercorns, bay leaf and about ½ teaspoon salt. Bring to the boil and cook vigorously for 15 minutes. Pour the stock through a sieve, mix with the cream and reduce by about half in an open saucepan over high heat.
In the meantime, heat 20 g butter in a pan and fry the mushrooms together with the garlic until the liquid has evaporated. Then remove the garlic clove.
Lightly salt the trout fillets, cut into bite-sized pieces and dust thinly with flour. Heat the remaining butter in a second pan and fry the fish pieces over medium heat for approx. 5 minutes, turning them frequently until golden.
Mix with the mushrooms and the boiled-down stock. Season with salt, pepper and mustard powder, heat to just before the boiling point and let it steep for approx. 5 minutes over a gentle heat, but do not boil any more.
Whisk egg yolks with vinegar and a little cream and stir into the fricassee. Add the basil to the fricassee just before serving.
Tip:
Instead of basil you can also use fresh tarragon or lemon balm.