Cut the bison into 1-inch cubes and place them in the freezer for 10 minutes to firm up for easier chopping.
In a small bowl, whisk together sherry vinegar, Dijon mustard, and egg yolk. Slowly drizzle in olive oil while whisking continuously until emulsified. Stir in shallots, capers, kosher salt, and all but 1 tablespoon of the parsley.
Hand chop the chilled bison to your preferred texture—fine or slightly coarse.
Gently fold the chopped bison into the dressing until evenly coated.
Using a 3 ¾-inch plating ring, shape the tartare on plates. Garnish with reserved parsley and lemon zest.
Serve immediately with toast points, chips, or crisp bread.