On a parchment lined baking sheet, toss the onions, carrots, and parsnips with the olive oil and salt until evenly coated. Arrange in an even layer and top with the thyme sprigs. Roast the vegetables in the oven until golden brown and caramelized, about 30 minutes, redistributing once.
Meanwhile, bring the quinoa and 2 cups water to a boil in a medium saucepan. Cover, reduce the heat to low, and cook for 15 minutes, or until the liquid is absorbed and the quinoa is tender. Season with salt and pepper.
Add the roasted vegetables to the pot and toss until combined with the quinoa. Transfer to a serving platter and drizzle with 1/4 cup tahini. Serve warm or at room temperature alongside the remaining tahini dressing.