Prepare the eel by rinsing and patting dry, then cut into 2–3 inch pieces so they cook evenly: Eel.
Place the eel pieces in a large saucepan and add cold Water to cover. Add a halved Onion, one or two Bay Leaf, a teaspoon of Black Peppercorns, a good splash of Malt Vinegar, and a pinch of Fine Salt.
Bring gently to a simmer over medium-low heat, skimming any scum, then reduce the heat and poach very gently for 45–60 minutes until the eel is tender and the cooking liquid is rich and slightly gelatinous. Do not boil vigorously.
Carefully lift the eel from the pot. When cool enough to handle, flake the flesh from the bones (remove any small bones) and return the meat to a clean dish. Strain the cooking liquor through a fine sieve or muslin into a bowl and discard the solids. Allow the strained stock to cool until it begins to set into a jelly.
While the eel is cooling, make the parsley sauce: melt Unsalted Butter in a small saucepan over low heat, then stir in Plain Flour to form a smooth roux and cook for 1–2 minutes.
Gradually whisk in warm Whole Milk a little at a time until the sauce is smooth and reaches a creamy consistency. Simmer gently for a few minutes to cook out the flour.
Stir finely chopped Parsley into the sauce, grate in a little Ground Nutmeg, and season to taste with Salt and freshly ground White Pepper. Add a small splash of the Malt Vinegar used earlier if you like a hint of tang, then taste and adjust seasoning.
To serve jellied eels, either pour some of the slightly set stock over the flaked eel and chill until fully set, or spoon warm parsley sauce over the warm eel—both are traditional. Garnish with a little extra chopped parsley and a grind of white pepper if desired.
