Preheat your oven to 350°F and lightly grease a 9x13 inch baking dish. While the oven heats, melt the butter in a large skillet over medium heat. Once melted, add the finely chopped onion and diced celery, cooking for about 10 minutes until softened and fragrant. Transfer this aromatic mixture to the prepared baking dish, spreading it evenly across the bottom to create your flavor foundation.

Spread the shredded cooked chicken evenly over the sautéed vegetables from Step 1, seasoning lightly with salt and pepper. In a bowl, whisk together the Bisquick and milk until just combined—the mixture will be thick and somewhat lumpy, which is exactly what you want. Gently pour this batter over the chicken layer without stirring; the batter will sink slightly and create those characteristic dumplings as it bakes.

In the same bowl you used for the batter, whisk together the chicken broth, cream of chicken soup, dried thyme, and a pinch each of salt and pepper. Carefully pour this broth mixture over the batter layer—again, resist the urge to stir. The magic of this casserole happens when you leave the layers undisturbed; the broth will cook the batter into tender dumplings while creating a savory sauce. I like to use low-sodium broth here so I can control the salt level throughout the dish.

Place the casserole in the preheated 350°F oven and bake uncovered for 45 minutes, until the top is golden brown and the broth is bubbling around the edges. Remove from the oven and let rest for 5 minutes—this allows the dumplings to set slightly and makes serving easier. Garnish generously with fresh chopped parsley just before serving for a pop of color and brightness.