Remove head and fins from cleaned fish. Scale if necessary, leaving skin and tail on. In large bowl mix crumbs, cheese and butter, adding salt to taste. In seperate bowl place 1/4 mixture, adding onion and egg/ Now add to remaining mixture, enough light cream to make a biscut-like dough. Place fish flat in shallow pan in which 1 tbsp. butter has been melted, stuffing it with onion mixture. Mould temaining mixture like a shell about fish, keeping its shape. Add enough light cream so there is about 1/2" in pan. Bake in moderate oven for about 1 hour, basting occasionally with cream sauce. Shoule be cris when baked. Serve hot, but left overs also good cold.