Instructions:
- Preheat oven to 375°F (190°C). Lightly grease a 9x13 inch casserole dish.
- Bring a pot of salted water to a boil. Blanch the broccoli florets for 2–3 minutes until bright green and slightly tender (al dente). Immediately plunge into an ice bath to stop cooking, then drain thoroughly.
- In a large saucepan, melt the 4 Tbsp of butter over medium heat. Whisk in the flour continuously for 1–2 minutes to form a smooth paste (roux).
- Gradually whisk in the warmed milk, ensuring no lumps form. Continue whisking constantly until the sauce thickens and coats the back of a spoon (about 5–7 minutes). Bring to a gentle simmer.
- Remove the pan from the heat. Stir in the cheddar, Parmesan, Dijon mustard, salt, and pepper until the cheeses are fully melted and the sauce is smooth. Taste and adjust seasoning.
- Gently fold the cooked rice and the drained broccoli into the cheese sauce until everything is evenly coated.
- Transfer the mixture to the prepared casserole dish. Toss the Panko breadcrumbs with the 1 Tbsp of melted butter and sprinkle evenly over the top.
- Bake for 30–35 minutes, or until the casserole is bubbly and the topping is golden brown and crisp.
- Let the casserole rest for 5–10 minutes before scooping and serving.