This honeynut squash recipe is bound to satisfy anyone at your Thanksgiving table. Once only available to chefs, honeynut squash (think: super-sweet butternuts) are now sold at farmers' markets and some grocery stores in the fall.
Place squash in a large pot and add water to cover. Cover and bring to a boil over high heat. Reduce heat to maintain a simmer and cook until tender, 15 to 20 minutes. Drain well and return to the pot; mash. Let cool for 15 minutes.
Preheat oven to 350°F. Coat a 9-by-13-inch broiler-safe baking dish with cooking spray.
Whisk whole eggs, evaporated milk, brown sugar, butter, cinnamon, 2 teaspoons vanilla and 1 teaspoon salt in a large bowl. Stir in the squash. Spread in the prepared baking dish.
Bake the casserole until the edges are starting to brown, about 45 minutes. Remove from oven. Turn the broiler to high.
Meanwhile, bring 1 inch of water to a bare simmer in a medium saucepan. Whisk egg whites, granulated sugar, corn syrup, sage, cream of tartar and the remaining 1/2 teaspoon vanilla and 1/8 teaspoon salt in the bowl of a stand mixer. Set the bowl over the barely simmering water and whisk until the sugar has dissolved, 2 to 3 minutes. Move the bowl to the mixer and beat with the whisk attachment at high speed until the mixture has doubled in size and is white and fluffy, about 3 minutes.
Spread or pipe the meringue over the casserole. Broil until lightly browned, watching carefully to prevent burning, 30 to 90 seconds.
To make ahead: Prepare through Step 3 and refrigerate for up to 2 days. Let stand at room temperature while the oven preheats. Refrigerate topping (Step 5) for up to 1 day.