Preheat the oven to 180°C (350°F).
Cut off the top of each apple and set the tops aside. Remove the inner seeds and core using an apple corer or a small knife.
In a bowl, place the walnuts and add the brown sugar and cinnamon. Mix well.
Place the apples in a heatproof dish and stuff them with the walnut mixture.
Add one tablespoon of butter on top of each apple, then cover them with the reserved apple tops.
Pour the water into the dish around the apples, then place it in the oven for about 30 minutes, until the apples are tender.
Serve the baked apples stuffed with walnuts warm.