Photograph: Jan Baldwin
To make the mayonnaise, whisk the egg yolk, vinegar and grainy mustard together in a bowl with an electric hand whisk or a balloon whisk. Slowly drizzle in the oil, whisking all the time, until the mixture has emulsified; season. Loosen the mixture with a tablespoon or two of water so it is thin enough to dress the salad.
To poach the trout, gently soften the onion and celery in a wide shallow pan in a little oil. Add the peppercorns, dill, wine and 250ml water. Bring to the boil and then remove from the heat. Place the trout fillets in the hot liquid, so they are completely covered, put a lid on the pan and leave to one side to go cold.
Toast the walnut pieces in a dry frying pan. Mix the chicory with enough mayonnaise to dress the leaves (about half). Take the trout from the poaching liquor with a slotted spoon and remove the skin. Flake the fish into the chicory and season. Divide between serving plates, scatter over the chopped walnuts and serve.
Get ahead: make the mayonnaise the day before; chill. Poach the trout and toast the walnuts a few hours ahead.
Kitchen secret: the mayonnaise recipe makes more than you will need but it also goes well with lamb or will keep in the fridge for a few days.
Note: this recipe contains raw eggs.