Preheat the oven to 400°F. Peel the bananas and slice in half lengthwise. Lay the pieces into a buttered 9x13x2-inch ovenproof dish. Pour the lime juice and rum evenly over the bananas. Sprinkle on the cinnamon, nutmeg, and brown sugar. Dot on the butter, if using, and bake for 10 to 15 minutes. Serve hot, topped with a tablespoon of fresh cream or ice cream.