Preheat oven to 375 F.
Cut squash in half and scoop out seeds. (The seeds can be roasted like pumpkin seeds.)
Place squash face-down in an oiled baking dish. Bake until tender when pierced with a knife, about 30 minutes. Remove squash from oven but keep oven on.
Prepare the stuffing while the squash is baking. Heat oil in a medium saucepan over medium heat. Add onions and sauté until onion is translucent. Add pistachios, dates, lemon zest, and cinnamon and sauté for another minute. Stir in the cooked quinoa and season to taste with salt and pepper.
Turn the squash upright in the baking dish and stuff with the quinoa mixture.
Cover dish with aluminum foil and bake for another 20 minutes.
Serve warm, garnished with extra pistachios or lemon zest, if desired. The peel of sweet dumpling squash is generally tender enough to be eaten.