Step 1

Spray a 6-qt. slow cooker with cooking spray. Slice onion in rings about 1/4 inch thick, then place in the slow cooker.

Step 2

Trim brisket of all but a thin layer of fat. Pat brisket dry with a paper towel and place, fat side up, in the slow cooker over the onions. Sprinkle garlic, thyme, salt, and pepper evenly over meat.

Step 3

Stir together broth, tomato paste, and Worcestershire sauce in a small bowl. Pour broth mixture around the meat.

Step 4

Cover slow cooker and cook on Low until meat is very tender when pierced with a fork, 6-8 hours.

Step 5

Transfer brisket from liquid to a platter. Using a slotted spoon, lift onions from liquid to a bowl. Loosely cover brisket and onions with aluminum foil and let stand about 10 minutes.

Step 6

Meanwhile, to make au jus to serve with the brisket, pour cooking liquid into a glass measuring cup and skim visible fat from the top. Pour liquid into a medium frying pan and bring to a boil over medium-high heat. Reduce heat to maintain a gentle boil and cook, uncovered, until volume has reduced by half, about 10 minutes.

Step 7

Slice meat across the grain. Arrange onions over the meat. Spoon some au jus over the meat and onions. Sprinkle with parsley. Pass remaining au jus to serve with the meat.