Untested - Sweetbread And Mushroom Pie

https://ifood.tv/pie/pie/pie/pie/102886-sweetbread-and-mushroom-pie

1 lb sweetbreads
2 Tbsps vinegar
1 tsp salt
Savory Pie Crust
1 cup Mornay sauce
(2 Tbsps butter
2 Tbsps flour
3/4 cups milk
1 egg yolk 
1/2 tsp salt
1/2 tsp chopped fresh thyme
dash tsp nutmeg
.75 oz gruyère, grated 
.75 oz parmesan, grated)
1 egg, well beaten
1⁄4 lb mushrooms, thinly sliced
Ground pepper to taste
1⁄4 cup grated parmesan
1 Tbsp egg wash

1. Soak the sweetbreads in cold salted water for 1 hour, then drain.
2. In a saucepan bring 4 cups of water, the vinegar, and salt in saucepan to a boil.
3. Add the sweetbreads, cover, and simmer for 20 minutes. Once cooked, remove with a slotted spoon and dunk in ice water to stop the cooking. 
4. When chilled, remove the outer membrane, cut out tubes and remove any fat. Cut into small cubes.
5. Roll out the pastry on lightly floured surface to 1/8 inch thick, then line a pie dish and pop into the refrigerator as you finish the next steps.
6. Preheat the oven to 425F with the rack on the middle.
7. To make the Mornay sauce, melt the butter in a saucepan and whisk in the flour. Cook until it's just starting to turn golden (to cook out the flour taste). 
8. Whisk in the milk in a steady stream; keep whisking until it turns into a smooth, thick sauce. Mix in the salt, pepper, thyme and nutmeg, then whisk in the egg yolk. Stir in the grated gruyère, and the parmigiana.
9. Beat egg quickly into Mornay Sauce.
10. Meanwhile, in a little butter, saute the mushrooms.
11. Once the Mornay Sauce is finished and the mushrooms have sweated, combine them along with the sweetbreads, salt and pepper and mix well.
12. Turn the filling into the pastry and sprinkle it with the cheese.
13. Pop the pie in the oven and bake, first for 10 minutes, then reduce the oven temperature to 350°F and bake and additional 20 minutes, or until pastry is golden brown.
14. Brush edge of the pastry with egg wash and bake an additional 10 minutes or until pastry is well browned.