Preheat the oven to 190 ° C top / bottom heat. Grease a roaster or casserole dish with butter.
Drizzle the trout with lemon juice, then season with salt and pepper. Roughly chop the mushrooms. Mix the onion, garlic, parsley and mushrooms together. Stuff the trout with the mixture. Place them side by side in the roaster and cover with flakes of butter.
Place the roaster in the oven on the middle shelf and cook uncovered for 30 - 40 minutes.
Remove the roaster and serve the fish with boiled potatoes.