**Prepare the Apples:**
Core each apple using a melon baller or small spoon, leaving about 1/2 inch at the bottom to hold the filling. Make thin, vertical cuts around the apple, being careful not to slice all the way through. Place the apples in the slow cooker.
**Mix the Cinnamon-Sugar Filling:**
In a small bowl, combine the granulated sugar, cinnamon, nutmeg, and vanilla extract. Spoon the mixture into the center of each apple, letting it fall between the slices.
**Make the Crumble Topping:**
In another bowl, mix the flour, oats, butter, brown sugar, and cinnamon until crumbly. Generously top each apple with the mixture.
**Cook:**
Cover the slow cooker and cook on low for 2–3 hours, or until the apples are tender but still hold their shape.
**Serve:**
Carefully remove the apples from the slow cooker. Serve warm, topped with vanilla ice cream, whipped cream, or a drizzle of caramel sauce.