For the CrustMix the flour, salt, and baking powder together in a food processor. Add the cubed butter and blend until it is a coarse meal. Have the machine running and add about ¼ cup of ice water slowly, a splash at a time until a dough forms, adding more or less water as needed.

Remove and shape into a disk. Wrap in plastic wrap and refrigerate for at least 2 hours.

For the FillingUsing only the white and light green parts of the leek, slice horizontally, and rinse between the layers to make sure it is very clean, then slice. Saute the leek in 1 Tbsp. butter until transparent in a medium sauté pan over medium-low heat.

Place into a large bowl. In the same pan, sauté the mushrooms in the other 1 Tbsp. of butter. Place into the large bowl with the leeks.

Once the chicken is cleaned and chopped place it also into the bowl with the leeks and the mushroom.

Bring a medium pot of salted water to a boil, blanch the frozen mixed vegetables and then remove them with a slotted spoon or wire ladle to the large bowl with the chicken/mushrooms. Add potatoes to the same water and cook for 6-8 min. until just tender.

Remove to the bowl. Adjust seasoning now to the chicken and vegetables with salt and pepper.

RouxMeanwhile, in a large heavy saucepan, melt butter and add flour. Cook stirring for 2 min. Add the thyme and rosemary. Slowly add in the chicken broth, now whisking, adding more if you find it too thick. It will thicken more in the pan with the crust.

Continue cooking for 10 min., whisking occasionally to keep it smooth and not burning to the bottom of the pan. Add salt and pepper to taste.

Preheat oven to 400F.  Spray a 9x13” pan with cooking spray. Add the filling and smooth out. Remove the crust from the fridge and roll out to 10x14” on a floured surface. Transfer carefully using the wooden rolling pin to cover the filling. Trim just to fit inside the pan. Brush the top with the beaten egg and water. Cut three 1 1/2” vents in the center of the pastry for steam. Sprinkle with sea salt.

Bake for 40-50 min. until pastry is golden and the filling is bubbling. Let rest for 5 min. and then serve.