01 - Preheat oven to 400°F. Lightly grease a 9x13-inch baking dish with butter or nonstick spray.
02 - In a large skillet over medium heat, melt 2 tablespoons of butter. Add the diced onion and cook until translucent, about 3 minutes. Stir in the minced garlic and cook for 1 additional minute until fragrant.
03 - Sprinkle 3 tablespoons of flour over the onion and garlic mixture. Cook, stirring constantly, for 1 minute to form a light roux and cook out the raw flour taste.
04 - Slowly whisk in the chicken broth followed by the whole milk. Continue stirring and cooking until the sauce thickens, about 2 to 3 minutes.
05 - Add the shredded chicken, thawed peas and carrots, corn, dried thyme, sea salt, and black pepper to the sauce. Stir everything together and let it simmer on low heat for 3 minutes. Pour the filling into the prepared baking dish.
06 - In a large bowl, whisk together 1½ cups flour, baking powder, garlic powder, sugar, and salt until evenly combined.
07 - Add the cold cubed butter to the flour mixture. Using a pastry blender or fork, cut the butter in until the mixture resembles coarse crumbs with pea-sized pieces remaining.
08 - Gently fold in the shredded cheddar cheese and chopped parsley. Pour in the milk and stir just until the dough comes together. Be careful not to overmix, as this will make the biscuits tough.
09 - Drop spoonfuls of the biscuit dough evenly across the surface of the chicken filling, covering as much area as possible.
10 - Bake for 25 to 30 minutes, or until the biscuit topping is golden brown and cooked through. The filling should be bubbling at the edges.
11 - While the cobbler bakes, combine the melted butter, garlic powder, and chopped parsley in a small bowl.
12 - Remove the cobbler from the oven and immediately brush the tops of the biscuits generously with the garlic parsley butter. Let the cobbler cool for 5 minutes before serving.