Preheat oven to 180°C (160° fan-forced). Line a large baking tray with baking paper.
Place corned beef, sugar, vinegar, bay leaves, cloves, peppercorns and onions into a large saucepan. Season with salt and cover with cold water.
Bring to the boil, reduce heat and simmer over low heat, partially covered, for 1 ½ to hours or until beef is firm to touch. Add carrots to pot for last 5 minutes of cooking time. Remove beef from pan and set aside until cool enough to handle, straining liquid into a bowl and reserving onions.
Meanwhile, place cabbage onto prepared baking tray, drizzle with oil, season and toss to coat. Roast for 25 minutes or until cabbage is tender.
Heat butter in a medium saucepan over medium-low heat. Once bubbling, stir in flour, remove from heat and then gradually stir in 2 cups cooking liquid. Return to heat and stir until sauce boils and thickens. Add cream and return to the boil. Stir in mustard and half the chives. Season to taste.
Slice beef thinly against the grain. Serve with mustard sauce, onions, carrots, cabbage and sugar snaps. Sprinkle with remaining chives.
Add vegetables to pan during last hour of cooking. Try whole baby potatoes, baby carrots, turnips or thick wedges of cabbage.
Corned beef is great cold on sandwiches with pickles and salad.