Dice the beef. Pat your beef dry with paper towels and place the beef on a plate. Transfer the plate to the freezer for 10 minutes. Finely dice the beef into 1/4" dice. Refrigerate while you prepare the rest of the ingredients.
Mix the sauce. In a mixing bowl, whisk together the egg yolk, dijon, red wine vinegar, and lemon juice until smooth. While whisking quickly, drizzle in the olive oil until emulsified and the sauce is thick. Season with salt and pepper to taste.
Chop the ingredients. Very finely mince the shallots and chop the cornichons and capers. Mince the parsley and chives.
Mix the tartare. Mix the diced beef, chopped shallots/cornichons/capers, sauce, and chopped herbs. Season to taste with salt and pepper.
Serve. Use a ring mold to mold the steak tartare on a plate and top with crispy onions, lemon zest, and more herbs. Serve with toasty sourdough bread.