In a 12-inch cast iron skillet, melt the butter over medium heat. Add the sliced onion and 2 tablespoons of water. Cook, stirring often, until lightly caramelized, about 12 minutes.
Add the diced celery, parsnip, butternut squash, red potato, sage, and thyme. Stir to coat, then add another 2 tablespoons of water. Cook for about 5 minutes, or until vegetables begin to soften.
Preheat the oven to 375°F.
Stir in the Better Than Bouillon Roasted Chicken Base, then add the flour and sour cream. Mix until smooth, then pour in the hot water. Stir in the diced chicken and frozen peas. Bring to a simmer and cook until the mixture thickens slightly, about 5–6 minutes.
Cut puff pastry into 2½-inch long strips, about 1 inch wide. Keep chilled until ready to use.
Remove skillet from heat. Lay puff pastry strips over the top in a criss-cross or rustic pattern. Whisk 1 tablespoon sour cream with 1 teaspoon water, and brush over the pastry. Garnish with extra thyme, sea salt, and cracked black pepper.
Bake for 25–30 minutes, or until puff pastry is golden and cooked through. Let cool slightly before serving.
