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 and onion, and sprinkle with sage and garlic powder. Cook until apple and onion have softened and chicken is cooked through, about 4 minutes. Remove from heat.
Once chicken is cool enough to handle, thinly slice and set aside with the apple-onion mixture.
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 cheese-covered side down. Press gently to seal.