Sweet-and-Sour Brisket

to 4 pounds beef brisket, fat cap trimmed to 1/4 inch thick

Kosher salt and freshly ground pepper

safflower oil

onions, thinly sliced (4 cups)

low-sodium chicken broth

chili sauce (such as Heinz) or ketchup

packed dark-brown sugar

distilled white vinegar

large carrots (about 1 1/2 pounds total), peeled and cut into 2-inch pieces

Yukon Gold potatoes, peeled and cut into 1 1/2-inch pieces

Flat-leaf parsley leaves, for serving (optional)