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.

Preheat oven to 325 degrees F. Line a 9 x 13 baking dish with foil.

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

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.

Place into oven and bake until tender, about 3 ½ - 4 hours; let stand covered.

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

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

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

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.

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