Clean the eels then cut into 2-inch pieces.
Dry on paper towels.
Heat the oil in a saucepan.
Add the onion and garlic and fry very gently until the palest gold.
Add the eel and fry until golden all over.
Add the lemon peel and sage, then put in the tomato concentrate blended with the water and wine.
Season to taste with salt and pepper then simmer gently, uncovered, for approximately 20 minutes or until the fish is tender and most of the liquid has evaporated.