Preheat oven to 220°C/200°C fan-forced. Arrange cabbage and onion on a large baking tray, leaving some space in centre for the sausage. Season and drizzle with 1 ½ tbs oil. Roast for 5 minutes.
Meanwhile, heat the remaining oil in a medium frypan over medium-high heat and cook sausage for 3 minutes each side. Transfer sausage to baking tray with cabbage. Add 1 tbs vinegar and 1 tbs maple syrup to pan, scraping bottom of pan with a wooden spoon, then pour over the cabbage and sausage. Roast for 25 minutes, or until cabbage is tender and outer leaves are crisp.
Combine mustard and chopped dill in a small bowl with remaining vinegar and remaining maple syrup. Season to taste. Drizzle over dressing and sprinkle with extra dill fronds to serve.
