How Long to Cook Beef Brisket Flat (Onion-Braised, for Slicing) in the slow cooker
Specification: 4 lb first-cut (flat) brisket trimmed to a ¼-inch fat cap, seared hard on both sides, laid fat-cap-up on top of 3 lb thickly sliced yellow onions with 2 tbsp tomato paste, 1 cup low-sodium beef broth, and 2 bay leaves — no liquid added over the top of the meat
3-Step Execution Protocol
Add beef brisket flat (onion-braised, for slicing) (4 lb first-cut (flat) brisket trimmed to a ¼-inch fat cap, seared hard on both sides, laid fat-cap-up on top of 3 lb thickly sliced yellow onions with 2 tbsp tomato paste, 1 cup low-sodium beef broth, and 2 bay leaves — no liquid added over the top of the meat) to the slow cooker. Set to LOW (9–10 hrs).
Cook for 9–10 hours on LOW. Keep lid on; do not stir unless specified.
A probe pushed into the thickest part of the flat slides in with no resistance — the classic "warm butter" feel; the onions underneath have collapsed into a jammy brown compote that sits in about 2 cups of dark liquid; the fat cap has rendered to a soft translucent ribbon. Confirm internal temperature reaches 200°F (probe-tender; comfortably above the 145°F FSIS whole-cut beef minimum). Rest for 30 minutes before serving.
USDA FSIS 145°F + 3-min rest whole-cut beef safe minimum; Certified Angus Beef "Braising Guide" (brisket flat reaches probe-tender collagen breakdown at 195–205°F internal); ATK "Slow Cooker Revolution" — "Slow-Cooker Brisket and Onions" (LOW 9–11 hrs, sear first, no liquid over the top); Cook's Illustrated brisket temperature/rest reference (chill-then-slice technique).