Preheat your oven to 350 F and grease a 2 qt. casserole dish with 1 tablespoon of butter.
Wash your fish and dry it with a paper towel.
Salt and pepper (1 teaspoon each) your fish, fold it in half, and add to the bottom of the casserole dish. Add parsley on top and pour wine on the bottom of the pan to surround your fish. Cover with parchment paper, then aluminum foil. Put in oven and cook for 15 minutes.
Sautee your mushrooms with 2 tablespoons of butter, half a cup of leeks, and half a teaspoon of salt for 5 minutes. When done, put it on the side and pour over the cream of shrimp soup. Stir everything together well.
Cook the shrimp in a roaring boil of water for 1 to 2 minutes, just until the color changes. Pull them out, drain them well, and put them on the side.
Take your fish out of the oven after the 15 minutes and pour a small cup of the wine reserve to the mushroom mixture. Mix together.
Pour the mushroom mixture on top of your fish and evenly spread.
Add your shrimp on top of the mushrooms next.
Lastly, add mashed potatoes on top of everything else. Spread it evenly.
Place in oven and cook at 425 F for 15 minutes. In the last 5 minutes, cook it on broil so you get a nice browning. Watch carefully.