Heat oven to 350°F Cut squash into quarters; remove seeds. In ungreased shallow baking pan, place pieces, cut sides down. Bake 45 to 50 minutes or until tender. Cool slightly.
Spray 1-quart casserole with cooking spray. Scoop squash pulp from skin; place in medium bowl. Discard skin. Add brown sugar, salt, cinnamon, 1/4 teaspoon nutmeg and butter to squash. Mix with potato masher or fork until well blended. Spoon into casserole. Sprinkle with chives and 1/8 teaspoon nutmeg.