Lay a large piece of aluminium foil or parchment paper flat on the counter. Scatter the celery tops, half the lemon slices, and parsley around the middle of the paper/foil. Then place the fillet of salmon on top.
Cover the salmon with the seasoning and top with the remaining lemon slices.
Use the parchment paper/foil to place the salmon in your slow cooker. Add the water and apple cider vinegar to the slow cooker, be sure to pour it to the side to ensure the seasoning is not washed off.
Cover the slow cooker with the lid and cook the salmon on LOW heat for 1-2 hours. Check the salmon with a fork after the first hour. If the salmon is not done, cover and let it cook for an addition 15-20 minutes then check again until it is done. Cut the salmon and serve over a bed of baby spinach.