Heat oven to 350 degrees. Cut four 2-foot lengths of foil each in half to make a 1-foot square. Place 1 fish fillet slightly below the middle of each square of foil. Sprinkle 1/4 of the carrot, celery, green onion, and parsley on each fillet. Top with lemon slices.
Fold the foil over the fish and crimp the edges together slightly. Place foil-wrapped fillets on a baking sheet and bake for 15-20 minutes or until the fish flakes easily.