If your rainbow trout are shop bought, they'll already be gutted and opened along the belly. If you catch them while fishing, you will have to perform that task. Make sure you rinse them well under the tap, inside and out. When you open the trout to wash it, you'll see a blood line along the backbone. Scrape that off before cooking, or it might affect the taste. Set your oven to approximately 130 C / 250 F.
Fill a large dish with tin foil, enough to make a parcel shape for your baked trout. You don't want your fish tightly bound. Lay your fish on the tin foil and fill with thinly cut lemon slices, salt, pepper and parsley.
Loosely enclose the parcel with the tinfoil and bake for 30 minutes.
Remove from oven, serve whole with salad or vegetables, garnish on plate with lemon and parsley.