Bring a skillet sprayed with nonstick spray to medium heat on the stove. Add chicken, season with salt and pepper, and cook for 4 minutes on one side.
Flip chicken in the skillet. Add apple, onion, sage, and garlic powder. Cook until the apple and onion have softened and the chicken is cooked through, about 4 minutes.
Meanwhile, lay bread slices flat and evenly spread the upward-facing sides with cheese.
Evenly top one slice with chicken and apple-onion mixture. Place the other bread slice on top with the cheese-covered side down. Press gently to seal. Set aside.
Remove the skillet from heat. Clean the skillet, if needed. Re-spray and return to medium heat on the stove.
Spread 1/2 teaspoon of butter on the upward-facing bread slice. Place the sandwich in the skillet with the buttered side down. Spread the remaining 12 teaspoon of butter on the upward-facing bread slice.
Cook the sandwich until hot and toasty, 1 to 2 minutes per side, flipping it gently.
Allow it to cool slightly. Then cut it in half, if you like, or just bite right in! Yummm . . .