In a heavy 2- to 3-quart saucepan, heat 1 cup of olive oil.
Add the onions, garlic, thyme, bay leaves, peppercorns, and salt and cook over moderate heat, stirring frequently, for about 7 or 8 minutes, or until the onions are transparent but not brown.
Pour in the vinegar and water, bring to a boil, reduce the heat and simmer the marinade uncovered for 20 minutes.
Meanwhile, brush the pieces of eel with lemon juice. Dip them into the flour and shake off the excess.
Then heat 2 Tblsp. of olive oil in a heavy 10- to 12-inch skillet until a light haze forms over it. Drop in 5 or 6 pieces of eel at a time and cook over moderately high heat for 4 to 5 minutes. Add more oil if needed.
Transfer the eel to a shallow 9- by- 12-inch glass, porcelain or enameled baking dish and pour over it the hot marinade.
When it is at room temperature, cover the dish with plastic wrap and refrigerate for at least 24 hours.
To serve, lift the pieces out of the marinade and arrange them on a serving plate. Moisten the eel with a little of the marinade before serving.