Ingredients:

• 1 flat-cut corned beef brisket (about 4 pounds)
• 1/4 teaspoon mustard seed
• 1/4 teaspoon coriander seed
• 1/8 teaspoon whole cloves
• 1/8 teaspoon whole peppercorns
• 2 to 3 bay leaves
• 1 large onion, peeled and quartered
• 6 Yukon Gold potatoes (about 1 pound), halved, or quartered, if large
• 1 small head cabbage (about 1 3/4 pounds), outer tough leaves removed and cut into 6 wedges
• 12 slender carrots (about 1 pound), peeled and left whole
• 2 teaspoons kosher salt
• 2 tablespoons chopped fresh parsley, for garnish
• Dijon or grainy mustard, for serving
• 1 cup sour cream
• 3 tablespoons whole milk
• 1 tablespoon lemon juice, or more to taste
• 1 tablespoon prepared horseradish in brine, or more to taste
• 1/2 teaspoon Dijon mustard
• 1/4 teaspoon salt
• 1/8 teaspoon ground black pepper
• 1/4 cup chopped fresh parsley