2 small acorn squash, halved lengthwise and seeded
Kosher salt and freshly ground pepper
3 tablespoons unsalted butter
1/2 onion, finely chopped
2 cloves garlic, minced
2 tablespoons all-purpose flour
2 cups milk
1 cup shredded cheddar cheese (about 4 ounces), plus more for topping
1/4 teaspoon ground nutmeg
2 cups cooked brown rice
1 10-ounce package frozen chopped spinach, thawed and squeezed dry
Chopped fresh parsley, for topping