Start by preheating your oven to 350°F (175°C).
In a large pot, bring water to a boil. Add the fresh broccoli florets and blanch them for about 2-3 minutes. Drain and set aside.
In a skillet over medium heat, melt 4 tablespoons of butter. Add the diced onion, diced celery, and sliced mushrooms. Sauté for about 5-7 minutes.
Sprinkle the flour over the sautéed vegetables, stirring constantly for about 1 minute. Gradually whisk in the milk, cooking until thickened, about 5-7 minutes.
In a large mixing bowl, combine the cooked rice, blanched broccoli, sour cream, and half of the shredded cheddar cheese. Pour the creamy sauce over the mixture and stir until evenly coated.
Grease a baking dish and pour the broccoli and rice mixture into it, spreading it evenly. Sprinkle the remaining cheddar cheese on top.
In a small bowl, combine the Panko crumbs with 2 tablespoons of melted butter. Mix and sprinkle over the cheese layer on the casserole.
Place the baking dish in the preheated oven and bake for 25-30 minutes until golden brown and bubbling.
Remove from oven and let sit for 5-10 minutes before serving. Enjoy warm!
