Remove beef from the fridge and place it in the freezer for 20–30 minutes while you prep the other ingredients.
In a medium bowl, combine chopped shallot, cornichons, caperberries, parsley, and Dijon mustard.
Remove the beef from the freezer. Slice into ¼-inch slices, then cut into strips, then cubes. Add to the mixing bowl and gently incorporate.
Add Worcestershire, olive oil, and a few dashes of hot sauce. Mix gently again to fully combine.
To serve, pack into a ring mold or shape gently by hand.
Top with an egg yolk and a spoonful of pickled mustard seeds if using.