Preheat the oven to 180°C (160°C fan-forced).
Line a small baking dish with baking paper. Make sure there's enough around the sides to cradle the vegetables and the cooking juices.
In a small bowl, whisk together the pomegranate molasses, vinegar, 2 tbsp of the olive oil, and a little salt.
Pack in the quartered onion and beetroots, then drizzle with whisked pomegranate molasses mixture.
Cover the dish tightly with foil and roast for 45 - 60 minutes. The beetroot should be soft when pierced with a knife. Set aside to cool for 10 minutes. Keep the juices.
Remove the outer skin from the onion quarters. Chop the beetroot and onion coarsely, then pour them into a large bowl, together with the cooking juices.
Add the walnuts and another 2 tbsp of olive oil. Mix with a hand-held blender.
Stir in the yoghurt and season to taste.
Garnish with chopped walnuts and mint, and serve immediately with crusty bread or toasted pita bread. Or, refrigerate in a sealed container until ready to serve.