Preheat the oven to 200°C. Top and tail the butternut and then cut it in half length ways. Remove the seeds with a spoon and then cut in half width-ways, and then cut each quarter into 2cm thickish wedges (you don't need to peel it).
Top and tail the onion, cut in half, peel and then cut each half into 3 wedges.
Place the veggies on a roasting tray and then drizzle with about 3 tablespoons of olive oil, then grind over some salt and pepper and coat everything well. Place in the oven and roast for about 30-40 minutes, be aware that you may have to take the onion out a little earlier so it doesn't get too crispy, and it might be good to turn the pumpkin over once while it roasts. You want the pumpkin to be melting soft and nice and browned around the edges.
While the veggies are roasting mix the tahini, crushed garlic and lemon juice in a bowl and give it a good stir. Add two tablespoons of water and stir until combined (tahini will first thicken when you add water). You want your tahini sauce to be thick, but drizzle-able, so add a little water (a teaspoon at a time if necessary).
Heat another tablespoon of olive oil in a small pan and throw in the pine nuts with a sprinkle of salt. Stir them regularly, and keep a sharp eye on them, as soon as they have started browning, take them off the heat (they can go from brown to black quite fast, so don't forget about them, not that I am talking from experience).
Roughly chop up the parsley.
Once the pumpkin and onions are cooked arrange them on a serving plate. Drizzle the tahini sauce over the top, then sprinkle over the pine nuts, parsley and za'atar.