Place the sliced onion in the slow cooker. Season the pork loin with salt and pepper and lay over onions.
In a small bowl, mix together the cranberry sauce and French dressing. Pour over the pork.
Place the lid on the slow cooker and cook for 4 hours on high, or 8 hours on low, or until the pork reaches an internal temperature of 145°F.
Pull the pork out and place on a cutting board to rest for 5-10 minutes.
Slice and serve topped with cranberry sauce.