Preheat the oven to 200°C. Place the beetroot in a roasting pan lined with baking paper. Spoon over 1 tablespoon of the oil, season. Place the onions in a single layer in a separate roasting pan. Spoon over the remaining oil and season. Place the onions and beetroot into the oven (onions above the beetroot). Roast for 25-30 minutes or until the onions are light golden and the beetroot is roasted. Set aside to cool.
Spread walnuts on an oven tray and cook for 5-8 minutes until toasted. Set aside to cool. Break into pieces.
Meanwhile, combine all the dressing ingredients in a medium bowl, season. Remove 3 tablespoons to a jug and set aside for serving. Add the grapes to the bowl and stir to coat in the dressing.
Heat a large non-stick frying pan over high heat until hot. Add the grapes and dressing from the bowl. Cook, shaking the pan often for 3-4 minutes until slightly blistered. Remove from the heat and set aside to cool in the pan.
Place the quinoa in a saucepan. Add 1 cup water and allow to stand for 15 minutes. Then place the saucepan over high heat and bring to the boil. Reduce the heat to low, cover and simmer for 12-15 minutes until the water has been absorbed. Remove from the heat, stand for 5 minutes then stir with a fork to separate the grains. Spread onto a tray to dry and cool for 10 minutes.
Spoon the quinoa onto a large platter. Top with the beetroot and onions. Spoon over the blistered grapes and pan juices, stir gently to combine. Top with walnuts and mint. Spoon over the reserved dressing, season and serve.
TipTip: If precooked beetroot in unavailable, peel and quarter 1 bunch fresh beets. Place in the centre of a large sheet of baking paper. Drizzle with oil and season, gather the edges of the baking paper and twist to secure. Place in a roasting pan and roast for 1-11/2 hours or until tender.