Rinse the trout, pat dry and season with salt inside and out. Peel the cucumber, cut in half lengthwise, remove the seeds and cut into thin slices. Rinse the scallions and cut into thin rings. Clean the mushrooms with a wet paper towel or kitchen cloth and cut into slices. Finely chop the dill.
Mix vegetables and mushrooms with dill and mustard. Season with salt and pepper. Add 40 grams of chopped butter (approximately 1 ounce) and fill the trout.
Preheat the oven to 250°C (approximately 480°F). Cut 4 pieces of alumnimum foil about 40 x 40 cm (approximately 16 x 16 inches). Sprinkle with 10 grams of butter and place the trout on the butter. Fold along the edges to seal into packets.
Place the packets in a grill pan or on a baking sheet and cook in the oven for 25 minutes, turning carefully once. Distribute the foil packets onto plates to serve.