1. Season the brisket liberally with salt and vacuum seal
2. Place in a container large enough to easily fit the brisket and cover with water by at least a couple inches. 
3. Remove the brisket and set the immersion circulator to 155°F. 
4. Once the water comes to temp, return the brisket to the container. The temperature will drop briefly but that’s okay. 
5. To keep your brisket from floating to the top, place something heavy on top of it. I use an enameled cast iron lid. Once your brisket is nice and cozy, cover the container with plastic wrap and cover with towels – this will help prevent the water from evaporating, and hold the temperature where it should be. Leave it alone for 36 hours, unless you need to add more water to keep it above the minimum water line marked on your circulator. 
6. After 36 hours, remove the brisket and dump the hot water. Return the brisket to the container and cool it down quickly with a lot of ice and cold water. (see note)
7. Once the brisket is cold, cut open the pour out the liquid – either into the braising sauce if you’ve already made it, or into a container if not. 
8. Slice the brisket 1/4 inch thick and place in a baking dish large enough to fit it as well as the braising liquid. 
9. Pour the braising liquid over the brisket. At this point, you can refrigerate the brisket overnight, or braise it right away. (I think it’s better if you let it sit in the fridge in the sauce overnight, but it’s great either way.)
10. Cook the brisket at 325°F for 1.5-2 hours. 

For the braising sauce: 

1. Sometime during the 36 hours, make the sauce: Place the porcini mushrooms in a heat proof bowl and cover them with 2 cups of boiling water. Set aside to rehydrate. 
2. Place a large high sided skillet or dutch oven over medium heat and add the olive oil. Once hot, add the onions and salt. Sauté for 12-15 minutes, or until golden brown and very soft. Adjust the heat as needed if they’re browning too quickly. 
3. Add the garlic and sauté another minute, until fragrant. 
4. Stir in the tomato paste and brown sugar and cook, stirring often, another 3-4 minutes, or until darker in color. 
5. Add the carrots, mushrooms and liquid you used to hydrate them. Simmer 5 minutes. Add the liquid from the brisket.