Trim pork loin of any excess fat. Season all over with salt and pepper. In a nonstick skillet, brown pork for on med-high heat about 4 minutes on each side until slightly browned on the outside.
Pour 1 cup apple cider into slow-cooker. Place 2 sprigs of rosemary in slow-cooker. Top rosemary with the pork and place remaining rosemary sprig(s) on top of pork.
Strain cooking liquid from slow cooker and skim off fat. Pour 2-1/2 cups cooking liquid into a saucepan.
Combine cornstarch and water until smooth. Stir cornstarch mixture into cooking liquid.
Bring to a boil. Cook, stirring constantly for 2 minutes or until thickened. Slice pork and serve with gravy.