1. In a pot, mix the syrup and cider. Bring to boil, lower heat and reduce by half.
2. In a food processor, finely chop the pistachios and maple sugar. Add flour and mix well.
3. Stir and add melted butter until large lumps are formed.
4. Spread the mixture on a baking sheet lined with parchment paper. Bake 10 minutes at 350°F or until slightly golden. Let cool.
5. Use a melon baller to remove the core and create a cavity in each apple, without piercing the apple.
6. Fill the cavity with the pistachio mixture and press down firmly.
7. Place the apples in an ovenproof dish and spoon the cider syrup over top. Bake at 350°F for 30 to 45 minutes, or more, depending on the size of the apples. While baking, spoon the cooking juice over the apples several times.
8. Serve as is, topped with cooking juice, or accompanied with a scoop of maple ice cream.