3 1/2 to 4 pounds beef brisket, fat cap trimmed to 1/4 inch thick
Kosher salt and freshly ground pepper
1 tablespoon safflower oil
2 onions, thinly sliced (4 cups)
2 cups low-sodium chicken broth
1 cup chili sauce (such as Heinz) or ketchup
1/2 cup packed dark-brown sugar
1/3 cup distilled white vinegar
4 large carrots (about 1 1/2 pounds total), peeled and cut into 2-inch pieces
1 3/4 pound Yukon Gold potatoes, peeled and cut into 1 1/2-inch pieces
Flat-leaf parsley leaves, for serving (optional)
Preheat oven to 400 degrees. Place brisket in a large, heavy-bottomed roasting pan; season with salt and pepper. Heat oil in a large straight-sided skillet over medium-high; add onions, 2 teaspoons salt, and 1/2 teaspoon pepper and cook, stirring, until onions are translucent, 3 to 4 minutes. Add broth, chili sauce, brown sugar, and vinegar; bring to a boil.
Place roasting pan over two burners; pour in broth mixture and return to a boil. Cover pan with parchment-lined foil and roast in oven 30 minutes. Reduce oven temperature to 350 degrees; roast 1 hour, 15 minutes more. Add carrots and potatoes and continue roasting, uncovered, tossing vegetables occasionally, 1 hour. Baste meat with pan juices and continue to roast, basting and tossing vegetables occasionally, until meat is tender but not falling apart, and potatoes and carrots are cooked, about 1 hour more.
Transfer brisket to a cutting board. Let stand 10 minutes, then slice and serve with sauce, vegetables, and parsley.