Heat a slow cooker on high until hot. Stir in the turkey fat, onion, carrots, celery, garlic, 3/4 teaspoon salt and 1/2 teaspoon pepper and cook, stirring occasionally until golden, 6 to 8 minutes. Stir in the turkey carcass, thyme, sage, and stock. Reduce the heat to low and cook the soup until the turkey meat is falling off the bones, 6 to 8 hours. Discard the turkey bones and season the soup with salt and pepper to taste. Serve sprinkled with the parsley leaves.