Preheat the oven to 220°C fan.
Place the pumpkin and onions in a large bowl with 2 tablespoons of oil, the sage leaves, ¾ teaspoon of salt and plenty of pepper. Mix well, then spread out on a large parchment-lined baking tray. Roast for 25-30 minutes, until cooked and golden-brown. Remove from the oven and set aside to cool for 10 minutes.
Meanwhile, cook the quinoa (if uncooked): fill a pot halfway with water, bring to a boil, and add the quinoa. Simmer over medium heat for 20 minutes until tender. Drain and let cool slightly.
Transfer the quinoa to a large bowl and mix with the lemon zest and juice, garlic, herbs, the remaining 1 tbsp olive oil, and ¼ tsp salt.
Add the pumpkin and onion to the quinoa and stir gently. Transfer to a serving bowl, drizzle with oil and serve.