Prep the broccoli - Wash and trim the broccoli, then chop it into bite-sized pieces. Steam the broc until it's a vibrant green, then plunge it into ice water to stop the cooking. Drain and spread in the bottom of your baking dish.
Preheat the oven and prep the mushrooms - Preheat the oven to 400℉. Sprinkle 2 T flour over the mushrooms and stir to combine. Slowly pour in the ½ cup of milk and stir until the mixture thickens. Remove from the heat.
Assemble the casserole - Pour the mushroom mixture over the broccoli. Spread the shredded mozz across the top. Melt 2T butter in the microwave. Combine the sourdough breadcrumbs and the Parmesan, then stir in the melted butter. Sprinkle the breadcrumb mixture across the layer of mozzarella.
Bake - Slice the casserole into the oven and bake until the cheese is melted and the breadcrumb mixture is beginning to brown.
Cool and Serve - Let cool slightly before serving.