When you make steak tartare, it's crucial to buy meat from a reputable butcher you know. We recommend beef fillet (tenderloin) for this recipe, and partially freeze it so that it's easier to chop.
Place the chilled beef medallions flat-side down on a clean work surface and, using a sharp knife, cut each into 1/4-inch¬-thick slices. Stack a few of these slices and cut lengthwise into 1/4-inch-wide strips. Turn the bundle 90 degrees and cut across the strips to make 1/4-inch dice and transfer to a medium bowl. Repeat with the remaining beef, returning the bowl to the refrigerator between batches. If pieces are any larger than 1/4 inch, return the diced beef to the work surface and coarsely chop to reduce the size. Cover tightly and chill for at least 15 minutes. Beef and other chopped ingredients can be prepared an hour or two ahead of time and kept separately, tightly covered in the refrigerator.
When ready to serve, gently combine the beef with the onion. garlic, cornichons, capers, anchovy, 2 tablespoons of the parsley, 1/2 teaspoon salt, the black pepper, and oil and return to the refrigerator, covered.
In a small bowl, mix together the mayonnaise, chile sauce, mustard, Tabasco, Worcestershire. and 1/2 teaspoon salt, stirring to combine thoroughly. Remove the beef mixture from refrigerator and gently fold into it all but 1 tablespoon of the mayonnaise mixture until just combined. Taste and add salt and any or all of the remaining mayonnaise mixture to your taste.
Place servings of steak tartare off-center on appetizer plates and sprinkle with the remaining parsley. Fan slices of toasted baguette alongside the beef and serve immediately.