Rinse the corned beef under cold water to remove excess brine. Place in a large heavy-based pot and cover with 2.5 litres of cold water.

Add bay leaves, peppercorns, mustard powder, and quartered onion. Bring to a boil over medium-high heat, then reduce to a gentle simmer. Skim any foam from the surface.

Simmer uncovered for 1 hour 45 minutes, maintaining a gentle bubble. The meat should be just tender when tested with a fork.

Add potatoes and carrots, and continue simmering for 20 minutes until just starting to soften.

Add cabbage wedges and simmer for a further 15 minutes until all vegetables are tender. Reserve 100 ml of cooking liquid in a jug before draining.

While the vegetables finish cooking, melt butter in a saucepan over medium heat. Sprinkle flour over the butter and whisk constantly for 1 minute to form a smooth paste (roux).

Gradually add the warm milk in three additions, whisking after each to prevent lumps. Whisk until smooth and thickened, about 3 minutes.

Stir in the reserved cooled cooking liquid, then add chopped parsley, salt, white pepper, and nutmeg. Taste and adjust seasoning. Keep warm over low heat, stirring occasionally.

Transfer the corned beef to a warm serving plate and arrange the cooked vegetables around it.

Pour the warm white sauce into a serving jug or boat and serve alongside, allowing each diner to spoon sauce over their meat and vegetables.

Rinse the corned beef and place in a slow cooker with 1.5 litres of water, bay leaves, peppercorns, mustard powder, and onion.

Cook on low for 6–8 hours. Add potatoes and carrots in the last 1 hour, and cabbage in the final 20 minutes.

Reserve 100 ml of cooking liquid, then drain. Make the white sauce as directed above.