Quarter onion and spread it in an instant pot. Add garlic cloves and seasoning mix.
Place brisket on top of onions, fat cap up.
Add beef stock.
Close the lid of the Instant Pot and make sure it's latched. Make sure to turn the valve to "seal."
Set Instant Pot to pressure cook on "stew/meat" setting for 75 minutes. It will take a few minutes to heat up and then start cooking.
Once time is up, let it naturally release for 20 minutes. After that, you can carefully turn the valve and do a fast release to let remaining pressure out.
Take brisket out and place it on the baking sheet lined with aluminum foil, fat cap up.
Set oven to high broil setting and set the rack in the middle of the oven.
Rub top and sides of the brisket with spicy mustard.
Place in the oven, under the broiler and let it caramelize until browned for a couple of minutes. (Make sure to keep an eye on it so the top doesn't burn.)
Take corned beef brisket out and make sure to slice the meat against the grain! Luckily, with the corned beef brisket, it's very easy to see the grain, you can look on the bottom since there is a fat cap on top.