Preheat oven to 200C. Combine beetroot and onions in a roasting pan with oil and thyme. Season to taste, then roast until beetroot is tender and onions are very tender (45-55 minutes). Set aside to cool briefly, then peel beetroot and onions, and quarter onions.
Spread walnuts on an oven tray and roast until golden and fragrant (3-5 minutes). Set aside to cool briefly, then break into pieces.
Meanwhile, cook quinoa in a saucepan of unsalted boiling water until tender (10-15 minutes). Drain and spread on a tray to dry and cool (5 minutes).
For tahini dressing, whisk ingredients in a bowl to combine, and season to taste.
Arrange beetroot and onion on a platter, scatter with quinoa and drizzle with dressing. Top with walnuts, dollops of yoghurt and mint, season to taste and serve.