In a large mixing bowl, combine the cooked spaghetti, shredded chicken, sliced mushrooms, and frozen peas.
In a separate bowl, mix together the cream of mushroom soup, chicken broth, and sour cream. Pour this mixture over the spaghetti and chicken mixture and stir until well combined.
Transfer the mixture to a greased baking dish. Sprinkle the shredded cheddar cheese evenly over the top.
In a small bowl, combine the breadcrumbs and melted butter. Sprinkle this mixture over the cheese.
Bake in the preheated oven for 30-40 minutes, or until the casserole is bubbly and the top is golden brown.
Let the casserole cool for a few minutes before serving.
Notes:
This casserole can be made ahead and refrigerated until ready to bake. Just add a few extra minutes to the baking time if baking from cold.