Slow-Cooker Corned Beef and Cabbage

INGREDIENTS

• 2 celery stalks, cut into 3-inch pieces
• 3 carrots, cut into 3-inch pieces
• 1 small yellow onion, cut into 1-inch wedges (root end left intact)
• 1/2 pound small potatoes, halved if large
• 6 sprigs thyme
• 1 corned beef brisket (about 3 pounds), plus pickling spice packet or 1 tablespoon pickling spice
• 1/2 head Savoy cabbage, cut into 1 1/2-inch wedges
• Grainy mustard, for serving