Combine stuffing mix, 1 1/2 tablespoons bourbon, 1 tablespoon syrup, chopped onion, and next 6 ingredients in a bowl; toss until well blended.
Unroll roast, and trim fat. Spread stuffing mixture over inside surface of roast. Reroll roast, and secure at 1-inch intervals with heavy string; sprinkle with 1/4 teaspoon salt.
Coat a large nonstick skillet with cooking spray; place over medium-high heat until hot. Add stuffed roast, browning on all sides. Place browned roast in a 4-quart electric slow cooker. Arrange onion slices and apple rings on top of roast. Combine 1 1/2 teaspoons bourbon, remaining 1 tablespoon syrup, and 1 cup apple juice; stir well, and pour over roast. Cover and cook on low-heat setting for 4 hours or until pork is done. Place roast, onion slices, and apple rings on a platter. Cut roast into 8 slices; cut onion slices and apple rings in half crosswise. Set aside, and keep warm.
Pour cooking liquid into a medium saucepan; stir in remaining 1 tablespoon bourbon and 1/8 teaspoon salt. Place flour in a small bowl; gradually add 1/4 cup water, stirring with a whisk until well blended. Add flour mixture to pan; cook over medium heat until thick and bubbly, stirring constantly. Serve gravy with roast, onion slices, and apple rings.