Heat the oven to to 220C/425F/gas mark 7. Put the squash and onions and any other vegetables in a large bowl, add three tablespoons of oil, a teaspoon of salt and some black pepper, and toss well.
Spread, skin down, on a baking sheet and roast for 40 minutes until the vegetables have taken on some colour and are cooked through. Keep an eye on the onions: they may cook faster than the squash, so may need to be removed earlier. Remove from the oven and leave to cool.
Put the tahini in a small bowl with the lemon juice, water, garlic and a quarter-teaspoon of salt. Whisk to the consistency of honey, adding more water or tahini as necessary.
To serve, spread the vegetables on a platter and drizzle over the sauce.