1. Preheat your oven to 375°F (190°C). Lightly grease a 9×13-inch baking dish with butter or olive oil to prevent sticking.
2. Bring a large pot of salted water to a boil. Add the broccoli florets and blanch them for 2-3 minutes until bright green and crisp-tender. Drain and immediately transfer to a bowl of ice water to stop the cooking. Drain again and set aside.
3. In a medium saucepan, melt the butter over medium heat. Add the chopped onion and cook gently for about 5 minutes until soft and translucent, stirring occasionally.
4. Stir in the minced garlic and cook for another minute until fragrant. Sprinkle the flour over the onion and garlic mixture, stirring constantly to create a roux. Cook for about 2 minutes, making sure it doesn’t brown.
5. Slowly whisk in the milk, stirring continuously to avoid lumps. Let this simmer gently for 4-5 minutes until the sauce thickens, then stir in the Dijon mustard if using. Season with salt, pepper, and thyme.
6. Remove the sauce from heat and stir in 1 cup of the shredded cheddar cheese until melted and smooth.
7. In a large bowl, combine the cooked rice, blanched broccoli, and cheese sauce. Mix gently to combine, making sure the broccoli is evenly distributed.
8. Pour the mixture into the prepared baking dish and spread it out evenly. Sprinkle the remaining shredded cheddar and grated Parmesan cheese over the top for that golden, crispy crust.
9. Bake in the preheated oven for 25-30 minutes, or until the top is bubbly and golden brown. You’ll know it’s ready when the edges are bubbling and the cheese is delightfully crusted.
10. Remove from the oven and let it rest for 5 minutes before serving. This helps the bake set and makes it easier to portion out.

One tip I learned from watching my mum over countless Sunday roasts is to resist the urge to dive in too soon—letting it rest makes all the difference in texture and flavour. Plus, it gives you a moment to pour a cuppa and savour the anticipation.