In a large pot, melt 3½ tablespoons unsalted butter over medium heat.
Add the celery, carrots, and onions. Sauté for 3–4 minutes until softened.
Add the minced garlic, thyme, and rosemary. Sauté for 2 minutes, stirring until fragrant.
Sprinkle in the flour and stir constantly for about 1 minute to create a roux. Let the flour cook out for two minuted
Pour in the chicken stock and heavy cream, stirring until smooth.
Add the shredded chicken, diced potatoes, and season with onion powder, garlic powder, celery seed, pink salt, and black pepper.
Simmer for 40 minutes, stirring occasionally, until the potatoes are tender and the sauce has thickened.
Stir in the frozen peas and adjust the seasoning with additional salt, oregano, onion powder, garlic powder, or smoked paprika, if needed.
Remove from heat.
Assemble
Pour the prepared filling into a 10-inch cast iron skillet or any oven-safe baking dish.
Set aside while you prepare the biscuit topping.