Preheat the broiler. In a food processor or blender, combine the squash, stock, oil and salt, and puree them. Taste for seasoning. Transfer the puree to a gratin dish, smoothing out with the back of a spoon. Sprinkle with cheese. Sprinkle with pistachio nuts. Place under the broiler just until the cheese is melted and the nuts are toasted — two to three minutes.
Recipe excerpted from Vegetable Harvest: Vegetables at the Center of the Plate by Patricia Wells (William Morrow Cookbooks, 2007).