Place on a baking sheet (it will bubble over—trust me). Bake 30–35 minutes until pastry is deep gold and puffed like a pillow, and the filling is bubbling at the edges.
Slowly pour in broth, then Guinness, scraping up any browned bits. Simmer 3 minutes until thickened. Stir in heavy cream, peas, parsley, and the chopped roasted chicken. Taste. Add more salt or pepper as needed.
Let rest 10 minutes. That’s the hardest part. Pull the skillet straight to the table. Crack the pastry with the back of a spoon. Let the steam roll out—close your eyes for a second. You’ll smell the Guinness, the thyme, the butter.
Reduce oven to 375°F (190°C). If your skillet isn’t oven-safe, transfer filling to a 9-inch baking dish. Unroll puff pastry and lay it over the filling, letting it drape over the edges slightly. Crimp with a fork if you’re feeling fancy. Cut 3 small slits for steam.