Rinse brisket under cold water, discarding the brine, then pat dry. Place in a large pot with the spice packet, quartered onion, smashed garlic and 6 cups water to cover. Bring to a boil.
3 lb corned beef brisket, 1 packet pickling spices, 1 large onion, 4 cloves garlic, 6 cups water
Reduce heat to low, cover and simmer for 2 hours until meat is fork-tender.
Add halved potatoes and carrot chunks; cook, covered, for 15 minutes until tender.
1 lb baby potatoes, 3 carrots
Stir in cabbage wedges; simmer 10 minutes more until cabbage is tender with slight bite.
1 small head cabbage
Horseradish Sauce
While the vegetables cook, whisk together sour cream, prepared horseradish, Dijon mustard and lemon juice until smooth. Season with salt and pepper to taste.