This hearty, warming casserole is both easy to prepare and very versatile — perfect for winter suppers!
You can add other root vegetables, such as parsnips, carrots or rutabagas, too; just make sure to slice the vegetables thinly for even cooking. A mandoline or food processor makes quick work of this chore.
Position a rack in the lower third of the oven and preheat to 350 degrees F. Oil a 2-quart baking pan and set aside.
Combine the flour, sugar, cheese, salt and pepper in a small bowl. Set aside.
Layer one-third of the squash, onions, potatoes and fennel in the prepared baking dish. Dust with one-third of the cheese mixture. Continue layering in this manner, finishing with the cheese mixture. Pour the milk into the pan and sprinkle the top with the Parmesan.
Bake until the vegetables are very tender and the top of the gratin has browned, about 1-1/2 hours. If the top browns too quickly, cover loosely with a piece of aluminum foil. Let the gratin rest 15 minutes before serving.