Beef

Trim large fat pieces from the brisket, score the fat in a cross-hatch, salt the brisket and let sit at room temperature 30 minutes
Pat the brisket dry and sear fatty side down in an ovenproof pot until browned, then brown the other side
Remove the brisket; add oil if needed, add chopped onions, sprinkle salt, sauté until lightly browned, stir in garlic and cook 1–2 minutes
Preheat oven to 300°F, tie the bay leaves with rosemary and thyme, nestle the brisket back in the pot, add beef stock and the tied herbs, bring to a boil, cover and transfer to the oven to cook 3 hours, flipping every hour
Add the carrots, cover and cook 1 hour more until the carrots are tender and the brisket is falling-apart
Remove the brisket to a cutting board, tent with foil and discard the tied herbs
Remove the carrots and half the onions and set aside covered, then purée the remaining pot contents until smooth and keep warm
Stir in mustard to the sauce if using
Slice the meat across the grain into 1/4–1/2-inch slices and serve with the onions, carrots, gravy and mashed potatoes