1-Getting Started: Prep Your Ingredients First, heat your oven to 375°F and set the rack in the middle for even baking. Dice the onion, peel and cut the carrots and celery into 1/2-inch pieces, mince the garlic, and chop the thyme leaves if using fresh. This prep work takes about 15 minutes and sets the stage for a smooth cooking process.

2-Cooking the Filling: Next, melt 4 tablespoons of unsalted butter in a large skillet over medium-high heat. Add the diced onion, carrots, celery, thyme, 1 1/2 teaspoons kosher salt, and 1/2 teaspoon black pepper, then sauté for 5 to 7 minutes until the vegetables soften and start to brown. Stir in the 1/4 cup all-purpose flour and minced garlic, cooking for 30 seconds to 1 minute until no dry spots remain, which helps thicken the sauce without lumps. Slowly pour in the 2 cups low-sodium chicken broth and 1 cup milk, scraping the bottom of the skillet to mix in all the flavors, and bring it to a boil. Reduce the heat to simmer and cook for 4 to 5 minutes until the sauce thickens enough to coat the back of a spoon. Shred or dice the 1 pound cooked boneless chicken, then add it along with the 1 cup frozen peas to the skillet, and season to taste with more salt and pepper if needed.

3-Assembling and Baking: Transfer the filling to a 9×13-inch baking dish and spread it evenly for a uniform bake. Arrange the biscuits from the 16-ounce package in a single layer over the filling to create that flaky crust. Bake for 20 to 30 minutes until the biscuits turn golden and the filling bubbles; check at 15 minutes and rotate the dish if needed, covering with foil if the biscuits brown too fast. Once done, melt the remaining 2 tablespoons of butter and brush it over the biscuit tops for extra flavor and shine. Let the casserole cool slightly before serving to avoid burns, and feel free to adapt with gluten-free biscuits if desired.