Place one cored apple in the center of a sheet of foil. Stuff a spoonful of the butter-sugar mixture into the hollow center, pressing it down gently. Add about 2 teaspoons of raisins or chopped walnuts on top of the filling, then finish with a little more of the butter-sugar mixture if there’s room. Repeat with the remaining apples and filling.
Wrap each apple tightly in its foil, bringing the sides up and around the apple and pinching at the top to seal. You want them snug so the buttery juices stay inside and steam the apples.
Arrange the foil-wrapped apples upright in a single snug layer in the bottom of a dry slow cooker, packing them close together so they help support one another.
Cover the slow cooker with its lid. Cook on LOW for 3 1/2 to 4 1/2 hours, or until the apples are very tender when pierced with a knife but still holding their shape. The exact time will depend on the size and firmness of your apples and the heat of your slow cooker.
Turn off the slow cooker and let the apples rest, still wrapped, for about 10 minutes. This helps the hot juices settle a bit so they don’t all spill out at once.
To serve, carefully lift each foil packet out of the slow cooker with tongs or a spatula. Place on small dessert plates or shallow bowls. Open the foil away from your face to avoid the burst of hot steam, then gently peel it back to reveal the apple and its spiced syrup.
Serve the apples warm, spooning any buttery cinnamon sauce from the foil over the top. Add ice cream or whipped cream if you like, and enjoy right away while the kitchen still smells like Sunday at Grandma’s.
