In a large cast iron pan melt 2 tbsp. butter over medium heat. Add onion and saute until well caramelized. About 15 minutes. Remove and set aside.
In the same pan melt 1 tbsp. butter. Turn off heat and add breadcrumbs, parsley and lemon juice. Stir until well combined. Remove from pan and set aside.
Layer potatoes in bottom of pan. Add cream of celery soup combined with an equal amount of water bake in preheated oven for twenty minutes.
Remove from oven and add layer of onions, then fish and top with breadcrumbs. Bake for another 20 minutes or until fish flakes easily with a fork. Season to taste, serve and enjoy.