Place the beef in the freezer for about 60 minutes, until it starts to form ice crystals and gets really firm but not frozen solid; you should still be able to pierce it with the point of a sharp knife.
Meanwhile, make the dressing: in a small mixing bowl, combine the olive oil, whole grain mustard, Dijon mustard, balsamic vinegar, sriracha (or sambal oelek), salt and pepper. Reserve.
When your beef has been in the freezer long enough, slice it thinly against the grain, then cut each slices into fine strips and then finally, dice those strips as finely as you can.
Finish grinding / chopping the meat with your knife until it has a coarse ground consistency, then place the meat in a mixing bowl.
Add the shallots, egg yolk, chopped parsley, chopped capers and reserved dressing to the beef and mix delicately until well combined.
Divide the mixture into 4 equal servings and press each serving into a round cookie cutter. Pack the meat down lightly with a fork and then remove the ring.
Garnish with micro greens and a drizzle of reduced balsamic vinegar, if desired.
Serve cold with fresh endives or toasted croutons.