Rinse Corned Beef Flat with water and pat dry.

Make several ¼ inch deep cuts in both sides of the brisket.

Add oil in 12-inch cast iron skillet or grill pan, heat over medium. Add Corned Beef Flat to skillet and cook until browned, about 3-5 minutes per side. Set aside.

Add onion slices to skillet and cook until browned over medium heat, about 5 minutes.

Place potatoes, carrots, and garlic in bottom of 6-quart slow cooker.

Place Corned Beef Flat in crock pot over vegetables.

Pour Beef Broth into crock pot.

Rub brisket with salt and pepper.

Add browned onions on top of Corned Beef Flat.

Cover with lid and cook over low heat for 9-10 hours, until fork tender and internal temperature reaches 145°F

Add cabbage wedges and cook an additional 1 hour or until cabbage is tender.

Place Corned Beef on cutting board, let stand for 15 minutes.

Strain and reserve 1-1/2 cups cooking juices from crock pot.

To make optional horseradish sauce, stir together reserved cooking juices from pot, cider vinegar, flour, sugar, and horseradish.

Cut Corned Beef against the grain into slices, in your preference of thick or thin.

Serve Corned Beef with vegetables. Top with Horseradish Sauce, if desired.