1. Preheat the oven to 375°F.
2. Prick the squash with the tip of a knife and place in a pan in the oven.
3. Bake until tender when pierced, about 50 to 60 minutes, depending on size.
4. Remove from the oven and, when cool enough to handle, cut in half. Scoop out the seeds and fibers and discard. Scoop out the flesh and pass it through a food mill placed over a bowl, or mash it with a potato masher. You should have about 3 cups purée.
5. Warm the olive oil or butter in a sauté pan over low heat. Add the onion and sauté until very soft and sweet, about 15 minutes.
6. Add the puréed squash, season with salt and pepper, and add the citron or lemon zest and cinnamon.
7. Cook over very low heat, stirring often, until dry, about 10 minutes. Transfer to a warmed serving dish and sprinkle with mint, parsley, or rosemary, if you like.