Meanwhile, start filling: In a large pot over medium heat, melt butter. Add onions, carrots, and celery and cook until vegetables are beginning to soften, about 10 minutes.
12 Stir in garlic, then stir in flour and cook until the flour mixture is golden and beginning to bubble.
13 Gradually whisk in chicken broth. Bring mixture to a boil and cook until thickened, about 5 minutes.
14 Stir in heavy cream, cubed chicken, peas, parsley and thyme.
15 Season mixture with salt and pepper.
Assemble pie
16 On a lightly floured surface, roll out one disc of dough into a large round about ¼” thick. Place in a 9-inch pie dish then add filling. Roll out second disc of dough into a large round about ¼” thick and place on top of filling.
17 Trim and crimp edges, then use a paring knife to create slits on top. Brush with egg wash and sprinkle with flaky sea salt.
18 Reduce heat to 375° and bake pie until crust is golden, about 45 minutes. Let cool for at least 15 minutes before serving.