To make spice rub, toast all ingredients separately, then grind to fine powder in a spice grinder.

Rub brisket with the spice rub and leave to sit for 1 hour.

Roughly chop carrot, celery, and 1 onion. Heat about tablespoon of oil in a large casserole over medium heat. Saute onion, carrot and celery until onion starts to brown, about 5 minutes. Add garlic and cook for 1 minute more. Remove from heat and set aside.

Cut brisket into four pieces. Heat a fry pan until hot, add about 1 tablespoon of oil, and cook brisket on each side until lightly browned, around 1 minute. Remove the browned brisket to the casserole with the vegetables. Deglaze beef pan with white wine, then add the deglazing liquid to casserole with chipotle and bay leaf. Add enough stock to cover beef, then cover with baking paper and bring to a boil. Turn down and simmer about 2 hours, or until meat can be pulled apart easily.

Remove beef from heat and set aside. Strain the cooking liquid into another clean pan and reduce until a thickened gravy has formed. Meanwhile, off the heat, shred the beef and return it to the casserole.

Slice remaining 2 onions and the flat mushrooms and saute in about 1 teaspoon of oil over medium heat until golden brown. Add mushrooms to shredded beef. Add reduced gravy back to casserole and mix well with the meat.

Wrap burritos and serve