Place onions, baby carrots, and celery in the bottom of your slow cooker; toss with olive oil.
Drain (don’t rinse) corned beef, rub with spice packet, and place on top of vegetables in slow cooker.
Whisk broth, mustard, malt vinegar, and brown sugar together; pour mixture over top of corned beef.
Cook for 8 hours on low.
Remove corned beef and vegetables from slow cooker; pour juices into sauce pot and bring to a boil. Boil for 10 minutes, allowing juices to reduce.
Dissolve cornstarch in water, pour into sauce pot, and stir for three minutes or until thickened.