Cut the eel into 3 to 4 pieces, wash and dry well. Fry the onions in the butter until golden. Add the eel pieces and brown lightly. Stir in the flour and cook until brown. Gradually stir in the cider, season with salt and pepper and cover the pan with a lid or foil. Bake in the oven for 1 hour. Serve the fish in the sauce, thickening it further with flour and butter if necessary.