Slow Cooker Corned Beef and Cabbage

Name: Slow Cooker Corned Beef and Cabbage
Ingredients: 1 1/2 pounds small redskin potatoes, halved; 4 carrots, cut into 2-inch pieces; 1 large onion, cut into 1/2-inch wedges; 2 stalks celery, peeled and cut into 2-inch pieces; 2 or 3 sprigs fresh thyme; One 4-pound piece corned beef brisket, rinsed; 12 ounces stout beer; 2 tablespoons pickling spice; 1/2 small head green cabbage, core intact and cut into thick wedges; 1/2 cup sour cream; 1/4 cup prepared horseradish; 2 tablespoons whole-grain mustard; 3 tablespoons unsalted butter; Kosher salt and freshly ground black pepper; 1/2 cup loosely packed parsley leaves, chopped