a 3 3/4-inch pastry ring, food processor (optional)
Cut the steak into 1-inch cubes and park in the freezer for 10 minutes.
Whisk the vinegar, dry mustard and egg yolks together in a small bowl. Whisk continuously while streaming in the oil until emulsified, then whisk in the shallots, capers, salt, and roughly 2/3 of the celery leaves and parsley.
Hand chop the meat to your desired texture. (Alternately, divide the meat into 4 batches and pulse each batch separately 3 to 4 times in the bowl of a food processor fitted with the standard S-blade.)
With clean hands, quickly fold the meat and dressing together. Plate using a 3 3/4-inch pastry ring and garnish with the reserved herbs and lemon zest.
Consumption of raw or undercooked eggs, shellfish and meat may increase the risk of foodborne illness.