Coat the inside of a crockpot with cooking spray.
Core and spiral-cut the unpeeled apples using an apple corer and spiral slicer, keeping each spiral intact.
3 large apples
Melt 7 tablespoons of the butter. Cut the remaining 3 tablespoons into three equal pats and set aside.
10 Tbsp salted butter
Combine the oats, flour, brown sugar, cinnamon and salt in a medium bowl. Add the melted butter and stir until evenly moistened and crumbly.
1 1/2 cups old-fashioned oats, 3/4 cup all-purpose flour, 3/4 cup brown sugar, 2 tsp ground cinnamon, 1/4 tsp salt
Gently compress each spiral-cut apple back into its original shape and place it upright in the crockpot.
Using a small spoon, fill the cored center of each apple with some of the crumble mixture. Scatter the remaining crumble around and between the apples. Spoon a little over each apple, if desired.
Place one reserved butter pat over the filled center of each apple.
10 Tbsp salted butter
Slowly pour 2/3 cup water down the inside edge of the crockpot, avoiding the apple tops.
2/3 cup water
Cover and cook on High for 2 hours. Insert a thin knife into an apple to check for tenderness. If it is not tender, cover and cook for an additional 30 minutes.
Check the surrounding crumble as well. If it is dry, pour 1/4 cup additional water over that portion, adding up to 1/2 cup if needed. If it looks floury and there is liquid in the bottom, gently stir only the surrounding crumble into the liquid and melted butter. Cover and cook for an additional 20–30 minutes as needed, until the apples are tender and the crumble is cooked through.
Carefully transfer the apples to serving bowls, supporting them from underneath. For 6 servings, divide each apple in half. Spoon the warm crumble and sauce around the apples and serve immediately with vanilla ice cream and caramel sauce, if desired.
Vanilla ice cream and caramel sauce