Heat the oven to 220°C/200°C fan. Add the cabbage, red onion, apples and garlic to a large roasting tray. Toss with olive oil, cumin, thyme, salt and pepper, then roast for 12 minutes. Add the cooked beetroot, toss everything together and roast for a further 8 minutes until the cabbage is softened and starting to caramelise.