For the fish, any boneless and skinless whitefish would work. Cut into bite size pieces. Pangasius would be ideal, but to change this up, go with 500 grams of pangasius and 250 grams of salmon OR even better yet, go with 400 grams of pangasius, 250 grams of salmon, and 100 grams of peeled, deveined and tail removed medium or large shrimp.

If you live in a country where fresh fennel is sold, use 1 small bulb of fresh fennel in place of the celery.

First, make the filling: In a large, deep-sided saute pan or Dutch oven, heat the butter over low-medium heat. Add the potatoes (can be dice with skin on or skin off), celery and onion, and stir to coat the vegetables with the butter. Cover the pan, and cook for 10 minutes, stirring occasionally, until the potatoes are just tender and the other vegetables are translucent.

Pour in the wine, and scrape up any stuck bits from the pan. Season with red pepper flakes, thyme, salt and black pepper.

Mix in the fish; it should begin to cook instantly from the heat in the pan. When the fish is half cooked, sprinkle on the flour, and stir to incorporate it. The fish and vegetables should get a bit clumpy as the flour cooks. This will take just a minute or two. When the flour is all incorporated (and no longer white), pour in the milk, and stir well. The sauce will begin to thicken as you bring it to a simmer.

When the fish sauce is thickened (like a nice white sauce), remove the pan from the heat, set aside.

Preheat your oven to 250 C. Get out a deep pie dish or 7x11 baking dish.

Transfer the fish and vegetable filling to a deep pie dish or small casserole dish. If your filling has been refrigerated, bring it to room temperature before adding the crust. Unroll the pie crust, and stretch it over the edges of the dish. Press the crust against the sides of pie dish, so there is a nice overhang. Trim off any extra. Make a few slits in the crust and brush with the egg and water mixture.

Set the pie on a rimmed baking sheet, and bake for 20 minutes, or until the crust is nicely browned. Remove from the oven, and set aside for 10 minutes to let the set up. Cut and serve.