With a food processor, slice the potatoes, rutabaga and squash 1/8 inch thick. Spray an 8 by 12-inch glass baking dish with cooking spray. Arrange half of the potatoes in the dish, overlapping them slightly and season with salt and pepper. Top with half of the rutabaga and the squash, seasoning each layer. Pepeat the layering. Pour the broth over and around the vegetables.
Cover tightly with foil and bake for 1 hour, until vegetables are almost tender. Remove the foil and pour the cream over the gratin. Sprinkle with panko and bake for about 30 minutes longer, until the liquid has thickened. Serves 8