Preheat oven to 375°F. In a 12 inch cast iron skillet over medium heat, melt 2 tablespoons of butter. Add the onion, carrots, and celery. Cook until the onion is translucent about 4-6 minutes. Add the minced garlic and cook for 1 minute more. Stir in the flour and cook for one minute, stirring constantly. Slowly add the chicken broth to the skillet, whisking constantly. Then whisk in the half and half, thyme, sage and rosemary and salt and pepper (to taste); bring it to a simmer. Cook for 4-5 minutes or until the sauce has thickened slightly.
Stir the shredded chicken and the frozen peas into the pot pie filling.
Arrange the uncooked flaky biscuits over the top of the sauce. Combine the 2 tablespoons melted butter and 2 tablespoons fresh herbs. Use a pastry brush or spoon to spread the herb butter mixture over the top of each biscuit.
Transfer the skillet to the oven and bake for 20-25 minutes or until the biscuits are golden brown and cooked throughout. Remove from the oven and let cool for 5 minutes before serving.