Pour the orange into the bottom of a 1.5-quart slow cooker.
Place the apple cut side up in the bottom of the slow cooker.
Put 1/2 teaspoon butter inside the core in each half of the apple.
Mix the brown sugar and cinnamon and sprinkle over the apple halves.
Cook on low setting for 2 hours until tender and soft.
Notes
Cooking slow and low will give you a beautiful creamy apple, so don't rush the cooking.
Not all apples are created equal, especially when it comes to baking. You'll want to choose a variety that holds its shape during cooking, such as Fuji, Honeycrisp, or Granny Smith.
Core the apples, and if desired, stuff the cavity with a mixture of spices like cinnamon, nutmeg, and sugar, or other fillings like nuts and raisins. You can also slice the apples if you prefer them that way.
While you don't need a lot of liquid in a slow cooker, adding a small amount (about a half cup) of apple juice, cider, or water will help steam the apples and infuse them with flavor.
BONUS TIPS: If you want a richer flavor, you might consider preparing a glaze or sauce with butter, sugar, and spices to pour over the apples during the last half hour of cooking.