Bring stock to the boil in a saucepan over high heat and cook until reduced by half.
Combine beetroot, vinegar, sugar and star anise in a large saucepan, add enough water to just cover and simmer, covered, over medium heat until just cooked (20-25 minutes). Do not overcook. Drain, cool, then thinly slice.
Melt butter in a saucepan, add onion and a pinch of sea salt and cook over low heat, stirring occasionally, until onion just starts to brown (10-12 minutes). Add beetroot and stir gently, add reduced veal stock and cook until beetroot is tender (5-6 minutes). Season to taste and serve with barramundi with braised oxtail.