Preheat the oven to 425°F/218°C adjusting the rack to the second-lowest position from the bottom.
Preheat a heavy-bottomed skillet over medium-high heat. Add the butter, then toss in the diced vegetables, carrots, celery, onions, potatoes and cook for 10 minutes. Stirring frequently.
Then add the crushed garlic, thyme, and rosemary leaves. Cook stirring for 30 seconds to release the oils in the herbs and garlic.
Add the flour, stirring until the flour is combined, and coats all of the vegetables and no dry flour remains.
While stirring gradually add the cream and chicken broth, cooking until the mixture thickens and bubbles reduce heat to medium-low and then add the shredded chicken. Allow the mixture to cook on low for 10 minutes. Remove from the heat, stir in the peas and allow to cool for 15 minutes. Taste and season with Kosher sea salt and freshly cracked black pepper, adjust for flavor.
While the Chicken Pot Pie Filling is cooling, prep your pie shells. You can use a 9-inch pie dish or individual servings in deep ramekins (as pictured). Fit the shortcrust pastry into the bottom of the vessel(s), spoon cooled filling into the bottom crust(s). Top with the second shortcrust pastry, flute the edges, (add garnish if desired with excess crust) then cut small slits in the top of the crust, which allows the steam to escape.
Place the Double Crust Pot Pies on a foil-lined baking sheet, brush crust liberally with whisked egg, and bake for 30-45 minutes until crust is golden brown.
Remove from oven and allow to cool for 15 minutes then serve by slicing if baking in a 9-inch pie dish or individual servings (as pictured).