Boil potatoes and leave to cool down. Cook green peas and douse with cold water. Dice onion.
Remove dark green from spring onions and cut the rest into stripes. Peel potatoes.
Mix well beef stock, vinegar, oil, mustard, salt and pepper. Cut potatoes into 12 cm slices and marinade them. Add onion, spring onions and green peas, blend carefully. Marinade all about 20 minutes.
Spice trout fillets with salt, pepper and lemon juice. Cover skin with flour. Fry fillets both sides, skin at first.