Cut the chicken into even 1-inch pieces. Add it to a 6-quart slow cooker with the frozen vegetables and both cans of condensed soup; stir to combine.
Cover and cook on LOW for about 3 hours or HIGH for about 2 hours, until the chicken reaches 165°F in the thickest pieces.
Whisk the baking mix and milk together; this makes a thin cobbler batter. Pour it evenly over the hot filling without stirring it in, then distribute the butter slices over the surface.
Cover and cook on HIGH for about 1–2 hours more. Begin checking the topping after 1 hour and continue until its center is set and a skewer comes out without wet batter. The covered cooker will not brown the top.
Spoon into bowls and serve hot. The timing shown allows 3 hours on LOW for the filling plus up to 2 hours on HIGH for the topping.