Make a mayonnaise by putting egg yolk, vinegar, mustard, and a little salt and pepper into a medium mixing bowl and whisking until thoroughly blended. Add ¼ cup of the oil drop by drop, whisking constantly. Gradually add the rest of the oil in a thin, steady stream, continuing to whisk as the mayonnaise thickens. Add one tablespoon of the lemon juice, or to taste, and adjust the seasonings. If the mayonnaise seems to be too thick, whisk in a little water to thin it.

Put ½ cup of the mayonnaise, reserve the rest for another use, into a small mixing bowl; whisk in Worcestershire and 1 teaspoon of the lemon juice, then enough milk to make a thin sauce that just to coat the back of a wooden spoon. Season to taste with salt and pepper add a little more Worcestershire sauce and or lemon juice if you like.

Making sure the beef is still very cold, slice into the thinnest possible sheets, using a razor-sharp, long bladed slicing knife or using an electric meat slicer.