Position a rack in the upper third of the oven; preheat to 425 degrees F. Put the squash, cut-side up, in a large microwave-safe dish with 1 tablespoon water; dot with 1 tablespoon butter and season with salt and pepper. Cover with plastic wrap and pierce with a knife to vent. Microwave until tender, about 15 minutes.
Meanwhile, heat the remaining 2 tablespoons butter in a large pot or Dutch oven over medium heat. Add the onion; cook, stirring, until softened, about 5 minutes. Add the garlic and cook 1 minute. Add the flour and cook, stirring, 3 minutes. Whisk in the milk until smooth, then bring to a simmer and cook until thickened, about 5 minutes. Add the cheese, stirring to melt. Add the nutmeg, 3/4 teaspoon salt and a few grinds of pepper. Stir in the rice and spinach.
Transfer the squash to a rimmed baking sheet and fill with the rice mixture. Sprinkle with more cheese and bake until lightly browned, about 15 minutes. Top with parsley.
Photograph by Charles Masters
Courtesy of Food Network Magazine