Rinse the potatoes thoroughly and cook in salted water for about 25 minutes.
Rinse the lettuce, trim, and spin dry.
Mix the vinegar with mustard, sugar, season with salt and pepper and drizzle in the oil.
Rinse the fish, pat dry, season with salt and pepper and sprinkle with lemon juice. Fill with coarsely crushed parsley.
Spread some flour on a shallow dish and roll the fish in it.
In a pan, heat the butter and cook the fish on each side for about 4 minutes until golden brown, remove, then arrange on plates.
Mix the lettuce with salad dressing and add the potatoes to the trout. Serve sprinkled with parsley.