Rinse the corned beef thoroughly and let it soak in cool water for 30-60 minutes.
Rinse again and place in a vacuum seal bag (or freezer bag if using the water displacement method - see post above) with the seasoning packet, beef stock and onion.
Seal and cook in the water bath for 48 hours.
Remove from the bag and pat dry.
Heat a cast iron skillet over medium high heat and add 1 tablespoon of cooking oil.
Brown the brisket for 1-2 minutes on each side and remove.
Slice and serve with cabbage and carrots optional).
How to Roast Cabbage and Carrots:
Heat the oven to 400F degrees.
Chop 1 head of cabbage into 8 pieces and place on a baking sheet.
Chop 6-8 carrots into 2" pieces and place on a baking sheet.
Drizzle the cabbage and carrots with 2 tablespoons of olive oil.
Sprinkle with 1 tablespoon of salt and 1 teaspoon of pepper.
Use your hands to massage the oil, salt and pepper into the vegetables.
Roast for 30-40 minutes, flipping everything over halfway through.
Serve with corned beef.
Notes
Make sure to rinse the corned beef and soak for at least 30 minutes to remove any excess salt before putting it in the water bath.
Make it ahead of time and store in the fridge for 1-2 weeks (in the bag) or freezer until ready to use. Heat for 30-60 minutes in a 145F sous vide water bath.
Make sure the corned beef stay submerged while cooking - otherwise you risk it cooking unevenly and can also promote bacteria growth in the bag. Use a sous vide sinker weight if your bag is floating.
You can also place the cooked corned beef under the broiler (or over a grill) instead of using a cast iron skillet to brown it.