My husband Meakin's version - from My Carolina Kitchen
Place one egg yolk in a bowl and add ½ teaspoon prepared horseradish, 2 anchovy filets, 1 teaspoon Dijon mustard, 3 dashes of Worcestershire sauce, 3 good shakes of hot sauce such as Tabasco, and mix well with a fork, mashing the anchovies as you go. Add ¾ of a pound of top quality freshly ground sirloin, ½ of a small white onion, finely chopped, 2 teaspoons drained small capers (chop if large), a couple of tablespoons of finely chopped parsley and mix well with a fork. Season the mixture with kosher salt and freshly ground black pepper to taste. Serve immediately on lightly buttered (use really good butter) slices of a crunchy French baguette or buttered toast points. If you like, top with thin slices of French cornichons or serve them on the side.
Cook’s notes: Ingredients are approximate. Add or subtract to taste. There are several ways to present the tartare – with the raw egg yolk on top, with the egg mixed into the meat along with the condiments, and with the condiments on the side for the individual dinner to incorporate into the meat.