Cook the long-grain white rice according to package directions. While the rice cooks, bring a large pot of salted water to a boil. Add the broccoli florets and blanch for 2-3 minutes until bright green and tender-crisp. Immediately drain and rinse under cold water to stop the cooking process. Set cooked rice and broccoli aside.

Preheat your oven to 375°F (190°C). In a large saucepan or Dutch oven, melt 4 tablespoons of butter over medium heat. Whisk in the flour and cook for one minute, stirring constantly, to form a roux.

Slowly pour in the warmed milk, whisking continuously to prevent lumps. Cook, stirring frequently, for 5-7 minutes until the sauce thickens. Reduce heat to low and stir in the heavy cream, Dijon mustard, garlic powder, onion powder, and cayenne pepper.

Remove the saucepan from the heat. Add the grated cheddar cheese one handful at a time, stirring until each addition is completely melted and smooth before adding the next. Once all cheese is incorporated, taste and season generously with salt and black pepper.

In a large bowl, gently combine the cooked rice, blanched broccoli, and the cheese sauce. Stir until everything is evenly coated. Pour the mixture into a lightly greased 9x13-inch baking dish and spread into an even layer.

In a small bowl, melt the remaining 2 tablespoons of butter and toss with the Panko breadcrumbs until evenly moistened. Sprinkle the buttered breadcrumbs evenly over the casserole.

Bake for 20-25 minutes, until the casserole is hot and bubbly and the topping is deep golden brown. Let it rest for at least 10 minutes before serving to allow the sauce to set.