Rinse the corned beef brisket thoroughly.

Place the corned beef in a large, heavy-bottomed pot or Dutch oven.

Pour onion soup over the corned beef.

Add whole peppercorns, minced garlic, bay leaf, and crushed rosemary to the pot.

Cover the pot tightly and cook over low heat for 3 and 1/2 hours (210 minutes).

Add the carrots, potatoes, and celery to the pot.

Place the cabbage wedges on top of the vegetables.

Cover and continue cooking for 1 hour (60 minutes), or until the vegetables are tender.

Remove the cooked corned beef, vegetables, and bay leaf from the pot.

In a separate bowl, whisk together flour and water until smooth, creating a slurry.

Slowly pour the flour mixture into the remaining sauce in the pot, stirring constantly to prevent lumps.

Cook the sauce over medium heat until it thickens.

Return the corned beef and vegetables to the pot with the thickened sauce.

Heat the mixture through until everything is warmed.

Serve hot.