Place white fish filets on a baking sheet with a little bit of oil and salt and pepper. Place in a preheated 375° oven and bake approximately 12-15 minutes or until fully cooked. Remove from the oven and cool enough to handle, then flake the fish with your hands or fork. Set aside. Begin making a thick white sauce for the pie. In a sauce pot melt butter and add the flour. Cook your roux to a nice smooth golden color and quickly whisk in your warmed milk. Cook to a nice thick and smooth “gravy”. Season to taste with salt and black pepper. Add a few grates of fresh nutmeg or at least ½ teaspoon worth and mix well. Taste for additional seasoning. While your white sauce is cooking, start working on the greens. If you will be using fresh turnip greens or spinach begin by making sure they are properly cleaned (grit free) and stemmed. Roll the greens together for a chiffonade cut or rough chop. Melt a few tablespoons of butter in a pan and cook the greens down. If you will be using leftover holiday greens, drain the greens from the stewing liquid as much as possible. You do not want a soupy fisherman’s pie. Start layering your fisherman’s pie. On the bottom of your favorite casserole dish add the cooked greens free of as much liquid as possible. Fold the flaked fish into your white sauce. Add the creamed white fish layer on top of the greens. Add the whole shrimp pieces on top of that, as much as you can, in a nice even layer. Finally top off the layers with your leftover mashed potatoes. If you do not have enough mashed potatoes, a quick mix of instant spuds and boiling water to stretch what you have is a great substitute. Smooth off the top of your fisherman's pie and, using a dinner fork, score the top in a “herring bone” or fish bone pattern using the tines of the fork. You will figure it out. Place the pie in a preheated 375° it oven for 35-40 minutes and bake until golden bubbly. Remove the pie from the oven and let it rest for 15 minutes before serving.