Open the brisket package and remove the seasoning packet. Pour it in a bowl with the extra spices if using them.
1 pickling seasoning packet
Once the air fryer is preheated, line the basket with aluminum foil. Take the brisket out of its package and rinse it with cold water. Pat it dry, and put it in the basket.
4-5 pound corned beef brisket *see note
Smear the mustard mixture over the top of the brisket. Sprinkle the spices from the bowl or seasoning package on top.
Air fry at 350 degrees F for 40 minutes. Pull out the basket and spoon some of the juices at the bottom on top of the brisket. If there aren't enough juices, sprinkle it with 1 tablespoon of water.
Air fry at 350 degrees F for another 15 minutes. When it is done, use an instant-read thermometer to verify that the internal temperature of the meat has reached 145 degrees F. If not, air fry it for another 5-10 minutes until done.
Use the aluminum foil to lift the air fryer corned beef out of the air fryer basket and put it on a serving dish. Let it rest for 10 minutes before slicing.
Notes
I have a 5.8-quart air fryer with a square basket, and it fits a 5 pound brisket. One with a round basket might only fit a 3-4 pound one.
Slice the brisket across the grain for maximum tenderness.
Don't preheat the air fryer with the foil in it.
Don't cover the brisket with the foil; just use the foil to line the air fryer basket. It will develop a nice bark on the outside if it isn't covered.
Basting the meat after the initial 40 minute cooking time helps ensure it doesn't dry out.