How to Make It
Stir together 1/4 cup water, sugar, vinegar, and salt in a small microwavable bowl. Microwave on HIGH until mixture comes to a boil, about 2 minutes. Stir to dissolve sugar. Place onion in a small heatproof bowl. Pour vinegar mixture over onion mixture. Let stand at room temperature 1 hour; drain. Set aside 1/4 cup drained pickled red onion for vinaigrette; reserve any remaining pickled red onion for another use.
Stir together all ingredients in a small container with an airtight lid. Attach lid, and chill at least 8 hours or up to 12 hours.
Wrap beef in plastic wrap, and chill in freezer 2 hours. Using a very sharp knife, cut chilled beef crosswise into 1/8-inch-thick slices (or as close to 1/8 inch as you can get). Stack beef slices, in batches, and cut into 1/8-inch-wide strips. Cut beef strips into 1/8-inch cubes. (Beef cubes can be wrapped in plastic wrap and chilled in an airtight container up to 2 hours.)
Place beef cubes, vinaigrette, and salt in a medium-size chilled bowl; gently fold together. Press plastic wrap directly on top of tartare mixture. Chill for 15 minutes to allow flavors to meld. Place a 3-inch metal ring mold on a chilled plate. Spoon a scant 1/2 cup beef mixture into ring mold; smooth top, and, using back of a spoon, make a small indentation in center of tartare. Remove ring. Carefully place 1 egg yolk in indentation. Repeat with remaining tartare mixture and 5 egg yolks. Sprinkle parsley evenly over tartare. Serve with crostini or crisped bread of your choice.