Part 1: Creating the Rich and Creamy Filling

1. Preheat and Prep: Position a rack in the center of your oven and preheat to 400°F (200°C). Have your oven-safe skillet ready on the stovetop.
2. Sauté the Aromatics: Melt the 3 tablespoons of butter in your 10 or 12-inch cast-iron or oven-safe skillet over medium heat. Once the butter is foaming, add the chopped onion, diced carrots, and diced celery. Sauté for 8-10 minutes, stirring occasionally, until the vegetables have softened and the onion is translucent. This foundational step, known as creating a “mirepoix,” is crucial for developing a deep, savory base for your filling.
3. Add Mushrooms and Garlic: Add the sliced cremini mushrooms to the skillet. Cook for another 5-7 minutes, until they have released their liquid and started to brown. Browning the mushrooms creates a deeper, earthier flavor. Stir in the minced garlic and cook for just one minute more until fragrant. Be careful not to burn the garlic, as it will become bitter.
4. Create the Roux: Sprinkle the ⅓ cup of all-purpose flour over the vegetables, along with the dried thyme, rosemary, salt, and pepper. Stir continuously for about 2 minutes to cook out the raw flour taste. This flour and fat mixture, called a “roux,” is what will thicken your sauce. The mixture will look pasty and coat all the vegetables.
5. Build the Sauce: While stirring constantly, slowly pour in the warmed chicken broth. Pouring it in gradually and stirring well will prevent lumps from forming. The mixture will thicken almost immediately. Once all the broth is incorporated and the sauce is smooth, slowly stir in the heavy cream or half-and-half until fully combined.
6. Simmer and Finalize: Bring the sauce to a gentle simmer and let it cook for 2-3 minutes, stirring, until it has thickened to a gravy-like consistency. It should be thick enough to coat the back of a spoon. Taste the sauce at this point and adjust seasoning with more salt and pepper if needed. Remember that the chicken and biscuits will also add flavor, so don’t oversalt.
7. Add Chicken and Frozen Veggies: Remove the skillet from the heat. Gently fold in the 3 cups of shredded cooked chicken and the 1 ½ cups of frozen peas and corn. Stir until everything is evenly distributed in the creamy sauce. The filling is now ready for its biscuit topping.

Part 2: Making and Baking the Cheddar Garlic Biscuits

1. Combine Dry Ingredients: In a medium-sized mixing bowl, whisk together the 2 cups of all-purpose flour, 1 tablespoon of sugar, 1 tablespoon of baking powder, 1 teaspoon of garlic powder, and ½ teaspoon of salt. Whisking ensures that the leavening agents are evenly distributed, which is key for a uniform rise.
2. Cut in the Butter: Add the ½ cup of cold, cubed butter to the dry ingredients. Using a pastry blender, two knives, or your fingertips, cut the butter into the flour until the mixture resembles coarse crumbs with some pea-sized pieces of butter still visible. These pockets of butter are what create the flaky, tender layers in the biscuits. Work quickly to ensure the butter stays cold.
3. Add Cheese and Buttermilk: Stir the 1 cup of shredded cheddar cheese into the flour mixture. Pour in the 1 cup of cold buttermilk and use a fork or spatula to mix until just combined. The dough will be sticky and shaggy. It is crucial not to overmix the dough, as this will develop the gluten and result in tough, dense biscuits.
4. Top the Pie and Bake: Immediately drop large spoonfuls (about 8-10) of the biscuit dough over the hot chicken filling in the skillet. Leave a little space between them as they will expand while baking. If using, brush the tops of the biscuits with the melted garlic butter.
5. Bake to Golden Perfection: Carefully transfer the heavy skillet into the preheated 400°F (200°C) oven. Bake for 20-25 minutes, or until the filling is hot and bubbly around the edges and the biscuits are tall, golden brown, and cooked through. You can test a biscuit for doneness by inserting a toothpick into the center; it should come out clean.
6. Rest and Serve: Remove the skillet from the oven and let it rest on a wire rack for 10-15 minutes before serving. This crucial step allows the filling to set and thicken slightly, preventing it from being too runny. It also brings the molten-hot filling down to a more manageable eating temperature.