Place apple and fennel in a 6-quart slow cooker; season with salt and pepper. Mix flour, 1/2 tsp. salt and 1/4 tsp. pepper in a shallow dish. Dredge pork in flour.
Warm oil in a skillet over medium-high heat. Cook pork, turning, until browned, about 10 minutes. Transfer to cooker.
Sauté onion 3 minutes. Stir in broth; bring to a boil. Cook until reduced by half, about 5 minutes. Reduce heat to low; stir in cranberry sauce. Pour over pork.
Cover; cook on low until pork is fork-tender, 6 to 7 hours. Transfer pork to a cutting board. Tent with foil; let sit for 10 minutes. Remove strings; slice pork. Arrange on a platter with apples and fennel. Skim fat from liquid. Season with salt and pepper. Spoon sauce over pork.
