How to Make It
In a bowl, mix 1/4 cup of the mayonnaise, 1 teaspoon of the parsley and the tarragon. Season with salt and pepper and refrigerate.
In a medium bowl, mix the meat with the 2 tablespoons of olive oil, the remaining 1 teaspoon mayonnaise, the mustard, Worcestershire sauce and capers. Season with salt and pepper. Transfer to a cutting board and finely chop the meat.
Transfer the tartare to cold plates. Season with salt and pepper. Sprinkle with the chopped onion and the remaining parsley. Drizzle with olive oil and serve with the herb mayonnaise, mustard and crostini.