Remove the corned beef brisket from its package. Rinse under cold water to remove excess brine, then pat completely dry with paper towels. (Rinsing is optional but reduces saltiness — see FAQ below.)
In a small bowl, mix together the mustard, apple cider vinegar, and honey or brown sugar to make the glaze.
Lay two large sheets of aluminum foil on your counter, overlapping slightly to make one wide sheet. Place the brisket fat-side up in the center.
Brush two-thirds of the mustard glaze all over the brisket — top, sides, and bottom. Sprinkle the contents of the pickling spice packet evenly over the top. Reserve the remaining glaze for basting.
Wrap the brisket tightly in the foil, sealing all edges to create a sealed packet. Place it in your air fryer basket. Make sure the foil does not block the top air vent.
Cook at 360°F for 1 hour. Carefully open the foil (watch for steam — use tongs), baste the top of the brisket with the reserved glaze, then re-seal the foil.
Cook for another 40 minutes at 360°F, or until a meat thermometer inserted into the thickest part reads at least 145°F. For more tender, pull-apart results, aim for 160–170°F. Add more time in 10-minute increments if needed.
Optional crisping step: Open the foil and cook uncovered at 400°F for 5 minutes to caramelize the surface.
Remove from the air fryer and let the brisket rest on a cutting board for 10 minutes before slicing. Always slice against the grain for the most tender results.