Prepare the Filling
Preheat the oven to 400°F and coat a 9x13-inch baking dish with nonstick spray.
In a large skillet over medium heat, warm the olive oil. Add the chopped onion and sauté for approximately 4 minutes until softened and translucent. Stir in the garlic and thyme, cooking for an additional minute until aromatic.
Incorporate the butter, stirring continuously to melt. Sprinkle in the flour and cook, stirring constantly, for about 2 minutes until the mixture forms a smooth paste.
Gradually whisk in the chicken broth, one cup at a time, ensuring the mixture thickens and remains lump-free. Stir in the heavy cream, kosher salt, and black pepper until fully combined.
Mix in the thawed vegetables and cooked chicken, ensuring an even distribution. Transfer the filling to the prepared baking dish and set aside.
Prepare the Biscuit Topping
In a medium bowl, combine the all-purpose flour, baking powder, sugar, garlic powder, baking soda, and kosher salt. Stir until well-blended.
Add the cold butter pieces, cutting them into the flour mixture using a pastry cutter or fork until the texture becomes crumbly with pea-sized bits of butter.
Mix in the shredded cheddar cheese and chilled buttermilk, gently folding until no dry spots remain. Adjust with additional buttermilk if necessary.
Assemble and Bake
Using a spoon or scoop, drop 12 portions of the biscuit dough evenly over the chicken mixture in the baking dish.
Bake in the preheated oven for 25–30 minutes, rotating the pan halfway through. The biscuits should be golden brown, and the filling should bubble around the edges.
Add the Butter Topping
While the cobbler is baking, mix the melted butter, Old Bay seasoning, and garlic powder in a small bowl.
Once out of the oven, brush the warm biscuits generously with the seasoned butter. Serve immediately.