Preheat oven to 350℉.
Place butter in the bottom of the casserole dish and place it in the oven while it's preheating to melt the butter.
Once the butter is melted, remove casserole dish from the oven and tilt the pan to make sure the butter coats the entire bottom of the pan.
Top the melted butter with an even layer of shredded chicken.
Top chicken with sliced mushrooms.
Top with a layer of frozen peas and carrots. Set casserole dish aside.
In a mixing bowl, combine the chicken broth, and cream of mushroom soup. Pour the mixture evenly over the chicken, mushrooms, frozen peas and carrots. Do not stir.
In the same mixing bowl combine the biscuit mix, including the butter seasoning packet, with 2 cups milk. Whisk to combine.
Pour this mixture evenly over everything in the casserole dish. Do not stir.
Bake for 45-55 minutes until hot and bubbly and the center is no longer jiggly.
Remove from the oven and top with shredded cheddar cheese.
Return to the oven and bake for an additional 5-10 minutes.
Allow casserole to rest 10 minutes before serving.