Preheat and prep. Preheat your oven to 190°C (fan-assisted). Lightly butter a 9x13 inch baking dish. Bring a large pot of salted water to a rolling boil. Drop in the broccoli florets and blanch for exactly 2 minutes. You'll see the florets turn a vibrant emerald green and a slight resistance when pierced with a knife. Drain immediately and plunge into a bowl of iced water to stop the cooking. Drain again and pat dry with a clean tea towel.

Build the foundation. In a large, heavy-bottomed saucepan, melt the butter over a medium heat. Add the diced onion and cook for 5-6 minutes, stirring occasionally, until it becomes translucent and softens, releasing a sweet, savoury aroma. Add the minced garlic and cook for another 30 seconds until fragrant — you'll smell it immediately.

Create the roux. Sprinkle the flour over the butter and onion mixture. Stir constantly with a wooden spoon for 1-2 minutes. You'll see the mixture turn into a pale, bubbling paste. This is your roux. It should smell nutty and look smooth, with no lumps of raw flour remaining.

Whisk in the liquid. Slowly pour in the milk in a steady stream, whisking continuously. Then add the chicken stock. Keep whisking as the mixture comes to a gentle simmer. You'll notice the sauce thickening and releasing a rich, creamy scent. Cook for 3-4 minutes, whisking now and then, until it coats the back of your spoon. Stir in the mustard powder, nutmeg, and a generous pinch of salt and pepper.

Incorporate the cheese. Remove the saucepan from the heat. Add the grated cheddar and half the Parmesan. Stir gently until the cheese melts completely into the sauce. The sauce will turn a pale gold and become silky and luscious. Taste and adjust the seasoning — you want it well-seasoned, as the broccoli and chicken will temper it.

Assemble the casserole. Place the blanched broccoli florets and shredded chicken into the prepared baking dish. Pour the cheese sauce over the top, ensuring everything is evenly coated. Use a spatula to gently fold the ingredients together so the sauce reaches every corner. The mixture should look creamy and inviting, with the green and white flecks visible.

Top for glory. In a small bowl, mix the panko breadcrumbs with the remaining Parmesan and the toasted flaked almonds. Sprinkle this mixture evenly over the casserole. For a final touch, dot a few small knobs of butter (about 15g) across the top — this will help the crust become deeply golden and crisp.

Bake and rest. Place the dish on the middle rack of the oven. Bake for 25-30 minutes, or until the sauce is bubbling vigorously around the edges and the topping is a deep, golden brown. You'll hear a gentle sizzle and see the top crust crackle. Remove from the oven and let it rest for at least 10 minutes. This rest time is non-negotiable — it allows the sauce to set slightly, making for cleaner, more elegant servings.