Spread the squash out on a large rimmed baking sheet, stir the garlic (and allspice, if you like) into the oil, drizzle it over the squash, toss it around to coat and roast for 30 minutes, or until soft and starting to turn golden. Set aside to cool.
To make the tahini dressing, whisk everything together and taste; add water to thin it if you like, or more lemon if you like it more lemony.
To assemble the salad, combine the squash, chickpeas, onion, and cilantro or parsley in a large bowl. Drizzle with dressing and toss to coat; serve immediately.