Start by preparing the crust. Combine flour, salt, and baking powder in a food processor. Pulse to thoroughly mix.
Add cubed butter and pulse until the dough looks like sand, with some pea-sized bits.
Next, drizzle in 1/2 cup ice water with the processor running. If the dough is too dry to come together, add more water only a teaspoon at a time.
Press the dough together with your fingers into a ball, wrap in plastic wrap, flatter into a disk and place the dough in the refrigerator to rest.
For the filling, preheat your oven to 400 °F.
Toss the chopped carrots, leeks*, and mushrooms in a tablespoon of olive oil and roast, 30 minutes.
Meanwhile, sauté the onion and celery in a large pot over medium heat in 2 tablespoons olive oil until softened, 3-5 minutes.
To the pot, add the chicken breasts, white wine, salt, black pepper, tomato paste, and 8 cups water. Bring the mixture to a boil, then reduce the heat to medium-low, cover, and simmer for 30-40 minutes, until the chicken is cooked through.
Once the chicken is cooked, remove the chicken from the pot. Add the roasted carrots, leeks, and mushrooms into the broth.
Pull the thyme and rosemary leaves from the sprigs, coarsely chop along with the sage, and add the chopped herbs to the broth. Bring the broth and vegetables back a simmer and cook, uncovered, until reduced by a few cups (about 40 minutes).
Meanwhile, shred the chicken breasts. Once the broth has reduced, return the shredded chicken to the broth, and add the peas.
In a separate sauce pan set over medium heat, melt the stick of butter. When the butter is fully melted, whisk in the flour until a smooth mixture is formed. Keep whisking, over medium heat, until the roux is golden in color, about 5 minutes.
Whisk the roux into the large pot of filling, bringing to a simmer until thickened (about 10 minutes).
When the filling has thickened, spoon it into a deep (at least 3 inches deep) 9”x13” baking dish.
On a lightly floured surface, roll out the crust until it is 1/4 inch thick, then cover the filling in large pieces of crust. I like to tear the crust dough by hand and layer over the filling in imperfect patterns, overlapping in some places, for a rustic look. (You can see this in the video above!)Any excess dough can be pinched around the edges of the baking dish, like a pie crust. In a small bowl, beat the egg and lightly brush over the dough. Sprinkle the top of the dough with kosher salt and cracked black pepper. Cut a few slits in the surface to let steam escape.
Bake until bubbly, about 45 minutes.
Serve in bowls, for a cozy night!