Lay out a large piece of tin foil. Slice a lemon and lay the slices on the foil. Lay over top the salmon fillet. Add butter, salt and pepper (or whatever spices you like) then wrap up the sides and fold tightly together, like a pouch. Place on a baking tray and cook in the oven for 25 minutes at 325° or 350° depending on how hot your oven runs.