Preheat your oven to 375°F (190°C) and lightly grease an 8x8-inch baking dish with butter or cooking spray.
Using a paring knife or apple corer, carefully remove the core from each apple, creating a well about 1 inch wide that goes almost to the bottom but doesn't pierce through it.
In a medium bowl, combine the brown sugar, rolled oats, softened butter, cinnamon, nutmeg, and chopped walnuts until the mixture is fully incorporated and crumbly.
Divide the oat filling mixture evenly among the apples, packing it firmly into the hollowed-out cores until slightly mounded on top.
Place the filled apples in the prepared baking dish and pour the 1/2 cup of water into the bottom of the dish around the apples to create steam.
Bake for 40-45 minutes, or until the apples are tender when pierced with a fork but still hold their shape, and the filling is golden brown and bubbly.