Work on a thoroughly cleaned cutting board with a very sharp knife. Trim the beef if needed, slice it into thin pieces about 1/4 inch thick, cut those into thin strips, then finely mince the strips to your preferred texture. Transfer the minced beef to a mixing bowl.

Add the mayonnaise to the bowl and season lightly with kosher salt and freshly ground black pepper. Stir to combine, then taste and gradually adjust the seasoning and flavor with tiny additions of olive oil, wine vinegar, or a touch of Dijon mustard until the balance suits you. If you like a deeper savory note, incorporate the fish sauce sparingly.

Fold in the minced shallot, chives, parsley, cornichons, and drained capers until everything is evenly distributed throughout the beef mixture.

Spoon or press the tartare onto a serving plate—use a ring mold for a neat presentation if desired. Nestle the egg yolk on top when using, and serve immediately with toasts and extra cornichons on the side.