Preheat oven to 375 F with rack in lower-middle position. Cut a thin slice from the top of each apple, then remove the core, scooping to within a half-inch of the bottom of the apple to create a well roughly 3/4-inch wide. Arrange the apples in a baking dish and firmly pack each apple with granola. Spoon 1 1/2 tsp maple syrup into each apple, then top each apple with a pat of butter. Pour water into the bottom of your baking dish and cover loosely with foil. Bake for 20 minutes, then remove the foil. Continue baking uncovered until the apples are soft and their skin is wrinkly, an additional 20-30 minutes. Serve warm with a scoop of yogurt, ice cream, or creme fraiche.