Preheat oven to 400°, Cut each squash lengthwise in half; remove and discard seeds. Using a sharp knife, cut a thin slice from bottom of each squash half to allow them to lie flat. Place in shallow roasting pan, cut side down. Add 1/2 in. of hot water. Bake, uncovered, until squash is tender, about 35 minutes. Drain water from pan; turn squash cut side up. Spread with butter; season with salt and pepper.
For filling, in a large bowl, toss apple pieces with lime juice. Add blueberries, walnuts and nutmeg; divide among squash halves. Drizzle with maple syrup. Bake until heated through, about 30 minutes. Cover loosely with foil if filling browns too quickly.
Test Kitchen tips
If blueberries aren’t in season, you can substitute almost any fresh berry.