Place each trout onto a square of aluminium foil. Open them up so that the flesh is facing upwards. On one side of each fish season with salt and pepper, half of the garlic, half of the chopped rosemary and half of the parsley. Top each fish with thin slices of butter, 3 rosemary sprigs and a few slices of lemon.
Place fish in the coals of a campfire or on a barbecue over a flame and cook for about 7 minutes on each side. If you can easily stick a fork into the fish, it is done. Cooking time will vary depending on the size of your fish and the fire. Allow to cool for a few minutes before opening to serve.
Squeeze one of the remaining lemon slices over each fish. Enclose the seasoning inside each fish and wrap securely with the sheet of aluminium foil. Wrap each fish in an additional piece of aluminium foil.