Steak Tartare

Name: Steak Tartare
Ingredients: 8 ounces high-quality beef tenderloin, any fat and gristle removed; 1 tablespoon finely chopped shallots; 1 tablespoon capers, drained and chopped; 1 tablespoon finely chopped curly parsley, plus sprigs for serving; 4 teaspoons Worcestershire sauce; 1 teaspoon fresh lemon juice; Flaky sea salt (such as Jacobsen's) and freshly ground black pepper; 1 large egg yolk; 2 tablespoons Dijon mustard; Lightly toasted pumpernickel bread, salted butter, and cornichons, for serving