Step 1

Place corned beef in a large bowl and cover with cold water; let stand 1-2 hours. Rinse with cold water and thoroughly pat dry.

Step 2

Preheat oven to 325 degrees F. Line a 9x13 baking dish with foil.

Step 3

MUSTARD MIXTURE: In a small bowl, combine mustard, allspice, cloves and 1 teaspoon pepper.

Step 4

Place corned beef onto the prepared baking dish. Spread MUSTARD MIXTURE evenly over one side of the corned beef; top with onions. Fold up all 4 sides of the foil over the corned beef, covering completely and sealing the packet closed.

Step 5

Place into oven and bake until tender, about 3 1/2-4 hours; let stand covered.

Step 6

Increase oven temperature to 425 degrees F. Line two baking sheets with parchment paper.

Step 7

Brush cabbage with 1 tablespoon olive oil. Place in a single layer onto the prepared baking sheet; season with salt and pepper, to taste.

Step 8

Place potatoes and carrots in a single layer onto the second prepared baking sheet. Add remaining 2 tablespoons olive oil; gently toss to combine. Season with salt and pepper, to taste.

Step 9

Place sheet pans into oven, on separate racks, and bake until cabbage is lightly charred and potatoes and carrots are tender, about 30-35 minutes, rotating pans and stirring halfway through baking.

Step 10

Thinly slice corned beef against the grain and serve with onions, cabbage, potatoes and carrots, garnished with parsley, if desired.