Preheat the oven to 210°C.
Dot butter over a large sheet of greaseproof paper on a baking tray.
Stuff one trout with herbs and lemon slices, season, and top with watercress.
Fold the paper edges to form a sealed parcel, using foil if necessary.
Repeat the process for the remaining three trout until four parcels are prepared.
Bake the trout parcels for 15 to 20 minutes.
Boil potatoes in salted water for 10 to 12 minutes, drain, and toss with butter.
Combine smoked trout, watercress, walnuts, and pears in a bowl.
Whisk vinegar, oil, and honey, season, and drizzle over the salad mixture.
Mix mustard, horseradish cream, and crème fraîche in a separate bowl.
Serve the baked trout with the buttered potatoes, salad, and horseradish cream.