Preheat and Prepare: Preheat your oven to 375°F (190°C). Grease a 9×13-inch baking dish or a similar 3-quart casserole dish.

Cook the Rice: In a medium saucepan, combine the uncooked rice and water or chicken broth. Bring to a boil, then reduce the heat to low, cover, and simmer for 15-20 minutes, or until the liquid is absorbed and the rice is tender. Fluff with a fork and set aside.

Blanch the Broccoli: While the rice is cooking, bring a large pot of salted water to a boil. Add the broccoli florets and cook for 2-3 minutes until they are bright green and slightly tender-crisp. This step is called blanching. Immediately drain the broccoli and rinse with cold water (or plunge into an ice bath) to stop the cooking process. Drain thoroughly and set aside. This ensures the broccoli doesn’t get mushy in the casserole.

Sauté the Onion: In a large pot or Dutch oven, melt 4 tablespoons of butter over medium heat. Add the chopped onion and cook, stirring occasionally, until it becomes soft and translucent, about 5-7 minutes.

Make the Roux: Sprinkle the flour over the softened onions and cook, stirring constantly, for 1-2 minutes. This cooks out the raw flour taste and forms a paste called a roux, which will thicken our sauce.

Build the Cheese Sauce: Slowly whisk in the warmed milk, a little at a time, ensuring each addition is fully incorporated before adding more. This prevents lumps from forming. Continue to cook, whisking frequently, until the sauce thickens enough to coat the back of a spoon (about 5-8 minutes).

Melt the Cheese: Remove the pot from the heat. Gradually stir in 2 cups of the shredded sharp cheddar and all of the Parmesan cheese until completely melted and smooth. Stir in the Dijon mustard, garlic powder, salt, pepper, and nutmeg (if using). Taste the sauce and adjust seasoning if needed.

Combine Ingredients: Add the cooked rice and the blanched broccoli to the cheese sauce. Gently fold everything together until the rice and broccoli are evenly coated in the creamy sauce.

Assemble the Casserole: Pour the mixture into your prepared baking dish and spread it into an even layer. Sprinkle the remaining 1 cup of shredded cheddar cheese over the top.

Prepare the Topping: In a small bowl, combine the Panko breadcrumbs with 2 tablespoons of melted butter and the paprika. Toss until the breadcrumbs are evenly moistened. Sprinkle this mixture evenly over the cheese layer.

Bake to Perfection: Bake, uncovered, for 25-30 minutes, or until the casserole is hot and bubbly and the topping is golden brown and crispy. Let it rest for 10-15 minutes before serving. This allows the casserole to set, making it easier to serve.