Preheat the oven to 325 degrees F.

Unwrap the brisket and place it fat side up inside a baking dish. If it came with a spice packet, sprinkle it over the top, then sprinkle on the black pepper and rub it in.

Cover the dish with heavy aluminum foil and place it in the oven for 2 1/2 hours.

After that, uncover and continue baking for another 30 to 45 minutes.

Check the brisket by inserting a fork in the meat; if it goes in easily, it is ready. If it meets with resistance, bake for another 30 minutes, or up to 1 hour if needed.

Once fork tender, remove the brisket from the oven and let it rest, covered loosely in foil.

Combine the vinegar and sugar (if using) in a saucepan over medium-high heat.

Bring to a boil, then reduce to medium-low and simmer until thick and reduced by half. Set aside.

Increase the oven temperature to 350 degrees F.

Cut the cabbage into 8 wedges.

In a skillet, heat olive oil over medium-high heat and add the cabbage. Sear it on both sides for about 1 minute per side.

Transfer the seared cabbage to a rack over a rimmed baking sheet, sprinkle with salt and pepper, and pour Guinness into the bottom of the baking sheet.

Bake for 20 minutes, or until tender and browned.

Slice or shred the rested beef and place it on a platter alongside the cabbage wedges.

Drizzle the balsamic reduction over the beef and cabbage for added flavor.