Preheat oven to 375°F (190°C). Lightly grease a baking dish large enough to hold the nectarine halves in a single layer.

In a small saucepan, combine the ruby port, granulated sugar, cinnamon stick, and orange zest. Bring to a simmer over medium heat, stirring until the sugar dissolves. Reduce heat and let it simmer gently for 10-12 minutes, until the liquid has reduced slightly and thickened to a light syrup. Remove from heat and discard the cinnamon stick and zest.

Place the nectarine halves cut-side up in the prepared baking dish. Top each half evenly with small knobs of butter and a sprinkle of brown sugar.

Carefully pour about half of the warm port reduction sauce over and around the nectarines. Bake for 20-25 minutes, or until the nectarines are tender when pierced with a fork and slightly caramelized.

Remove from the oven. While the fruit is still warm, spoon the remaining port reduction sauce over the baked nectarines.

Serve immediately while warm, optionally topped with a dollop of mascarpone cheese or a small scoop of vanilla ice cream.