Preheat your oven to 375°F and lightly grease a 9x13-inch baking dish.

In a large pot, steam or blanch the broccoli florets until they are slightly tender, about 3-4 minutes. Drain and set aside.

In a large saucepan, melt the butter over medium heat. Once melted, stir in the flour to create a roux. Cook the roux for about 1-2 minutes until it becomes lightly golden.

Gradually whisk in the chicken broth and heavy cream, making sure to remove any lumps. Bring the mixture to a simmer and cook until it thickens and becomes smooth.

Stir in the grated Parmesan cheese, Dijon mustard, garlic powder, onion powder, salt, and black pepper. Mix until well combined and the cheese is fully melted.

Add the cooked and shredded chicken, broccoli florets, and sliced mushrooms to the sauce. Gently stir until all the ingredients are evenly coated with the creamy sauce.

Transfer the mixture to the prepared baking dish and spread it out evenly. Cover with the cheddar cheese.

In a small bowl, combine the breadcrumbs and melted butter. Evenly sprinkle this mixture over the chicken and broccoli in the baking dish.

Cover the baking dish with aluminum foil and bake for 20 minutes.

After 20 minutes, remove the foil and continue baking for an additional 10-15 minutes or until the top is golden and bubbly.

Once done, let the Chicken Divan rest for a few minutes before serving to allow the flavors to meld together.