Put the sliced onions in a large saucepan with the fish, cut in 3 inch lengths, the chopped herbs, garlic and a little salt. Cover with the wine and stock or water, put the lid on the saucepan, bring to the boil, and boil moderately fast for 15 minutes. Remove the pieces of eel from the liquid, and strain the latter through a sieve into another saucepan. Let it boil till reduced by one-third. Now add the butter and flour, which should have been well worked together, the mushrooms, previously cooked, and the onions, cooked in butter to a golden colour. Cook for a few minutes longer. To dish up, pile the pieces of eel in the centre of the dish, pour the sauce over them, and garnish with the mushrooms, onions and croûtons of fried bread.