Heat 1 tablespoon of oil in a large sauté pan over medium-low heat and cook the garlic until it is lightly golden and fragrant, taking care not to burn it. Transfer it to a high-walled mixing cup or container that can accommodate the head of an immersion blender. Set aside.

Increase the heat to medium, add 2 cups of onions to the pan, season them with a couple pinches of salt, and cook until they are translucent and lightly browned on the edges. Scrape into a bowl and set aside.

Increase the heat to medium-high (but closer to high), and add another tablespoon of oil and the remaining cup of onions. Cook the onions until they are burnt, but still tender in spots.

Remove the burnt onions from the pan and add them to the measuring cup with the garlic.

Add 2 more tablespoons of oil to the pan, then add the carrots and fennel. Season with a couple of pinches of salt, and cook until they are browned and blistered on outside, but not soft. Transfer to the bowl with the translucent onions. Add the bay leaf to the bowl, and toss to mix. Transfer the vegetable mixture to a sous-vide bag (or a freezer bag). Sear the fat side of the brisket until it is golden-brown and add it to the bag with the vegetables.

Add the mustard, Worcestershire sauce, wine, and onion soup packets to the mixing cup with the burnt onions and garlic, and puree everything together with an immersion blender (this can also be done in a 'regular' blender). Pour the mixture over the meat and use your hands to spread it over the entire hunk of brisket.

Seal the bag with a vacuum sealer if you are using vacuum bags; if you’re using freezer bags, double bag it with a second one, express any extra air using the water displacement method, and clip the top of the bag to the side of the bath. Cook the brisket for 18 hours in a water bath set to 185℉. (Cover the bath with plastic wrap to keep the water from evaporating.)

Once the cook time has elapsed, remove the brisket from the bath and decide if you want to eat it right away or serve it later. If you’re going serve it later, place the whole bag in an ice bath to chill to room temperature, then transfer to the fridge until you are ready to serve.

If you’re going to eat it right away, transfer it to a serving platter or pan, and carve it against the grain into thin slices. You may find this to be difficult, given how tender the brisket is; don’t worry about it too much. Pour the remaining contents of the bag into a colander to separate the jus from the vegetables.