Heat the oven. Preheat to 350°F (175°C).
Lightly grease a medium baking dish or line it with parchment for easy cleanup.
Prep the apples. Wash and halve the apples from stem to bottom. Use a spoon or melon baller to scoop out the core and seeds, leaving a shallow cavity for the crumble. Place apple halves cut-side up in the baking dish.
Mix the crumble. In a bowl, combine oats, almond flour, chopped nuts, cinnamon, nutmeg, and a pinch of salt.
Stir in melted butter, applesauce, maple syrup (and brown sugar if using), and vanilla. The mixture should be clumpy and slightly moist.
Fill and top. Spoon the crumble generously into the apple cavities and mound extra on top. Press lightly so it adheres.
Add liquid to the dish. Pour water or apple cider into the baking dish around the apples.
This creates steam to soften the fruit and prevents scorching.
Bake. Cover loosely with foil and bake for 20 minutes. Remove the foil and bake another 15–20 minutes, until apples are tender when pierced and the topping is golden and crisp.
Rest and serve. Let the apples sit 5–10 minutes so the juices settle. Serve warm with a dollop of Greek yogurt or a small scoop of ice cream if you like.