Preheat the oven to 400° F.
Put the squash and onion in a large mixing bowl, add 3 tablespoons of the oil, some roasted garlic, 1 teaspoon salt, some black pepper and toss well.
Spread on a baking sheet with the skin facing down and roast in the oven for 30 to 40 minutes, until the vegetables have taken on some color and are cooked through. (Keep an eye on the onions as they might cook faster than the squash and need to come out a bit earlier)
Remove from the oven and leave to cool.
To make the sauce, combine the tahini in a small bowl along with the lemon juice, water, garlic, and ¼ teaspoon salt. Whisk until the sauce is the consistency of honey, adding more water or tahini if necessary.
Pour the remaining 1 ½ teaspoons oil into a small frying pan and place over medium-low heat.
Add the pine nuts along with the ½ teaspoon salt and cook for 2 minutes, stirring often, until the nuts are golden brown.
Remove from the heat and transfer the nuts and oil to a small bowl to stop the cooking.
To serve, spread the vegetables out and drizzle the tahini sauce over. Sprinkle the pine nuts and their oil on top, followed by the za’atar and parsley