Prep the oven. Heat oven to 400°F.

Cook the bacon. Heat a large oven-safe skillet or Dutch oven over medium heat. Add the bacon and cook until crispy, about 5–7 minutes, turning occasionally. Use a slotted spoon to transfer the bacon to a paper towel–lined plate, leaving the rendered bacon fat in the pan.

Sauté the vegetables. Add the onion, carrots, and celery to the bacon fat and sauté for 5–6 minutes, until softened. (If the pan seems dry, add a drizzle of olive oil or a small knob of butter.) Stir in the garlic, thyme, and rosemary and cook for 1 minute until fragrant.

Build the sauce. Stir in the Dijon mustard, then sprinkle the flour evenly over the vegetables. Cook, stirring constantly, for 1–2 minutes. Gradually pour in the chicken broth while stirring, scraping up any bits from the pan. Add the milk and bring to a gentle simmer. Cook for 4–5 minutes, stirring often, until the sauce thickens to a creamy, stew-like consistency.

Finish the filling. Stir in the shredded chicken, peas, Parmesan, cooked bacon, salt, and pepper. If you would like a soup-ier filling, you can add in an extra splash of milk. Taste and add additional salt and pepper if needed. 

Add the puff pastry topping. Unfold the mostly thawed puff pastry and cut it into 2-inch squares or rectangles. (It’s easiest to cut if it’s not completely thawed.) Arrange the pieces evenly over the hot filling, slightly overlapping them so you get those pretty, layered edges. Brush lightly with egg wash, then season with a few twists of pepper and salt.

Bake. Transfer the entire skillet to the oven and bake for 25–30 minutes, or until the puff pastry is deeply golden and the filling is bubbling around the edges. (If any spots of pastry start to brown too quickly, tent loosely with foil.)

Serve. Let the pot pie cool for 5–10 minutes, then sprinkle with extra parsley if you’d like. Scoop into bowls, making sure everyone gets plenty of the flaky pastry on top, and enjoy!