Pat brisket dry, then rub with salt and pepper. Sear in a large skillet heated to medium-high for 4 to 5 minutes, just to brown the outside edges.
Place the coarsely chopped onion, celery, and garlic in the bottom of the slow cooker. Add the whole peppercorns, bay leaf, and beef broth.
Whisk together mustard, Worcestershire sauce, olive oil, lemon juice, honey, and cayenne.
Rub the mustard mix onto the brisket, then place in the slow cooker on top of the vegetables. Cover and cook on low for 8 to 10 hours, or until the brisket is tender.
Remove from slow cooker and let rest at least 20 minutes. Strain the liquid and reserve, keeping warm until ready to eat. Be sure to slice against the grain to get maximum tenderness.