Poach the chicken: Place the chicken breasts in a medium saucepan and cover with cold water. Bring to a gentle simmer – you'll see small bubbles breaking the surface, not a rolling boil. Cook for 15-18 minutes, until the chicken is firm to the touch and the internal temperature reaches 74°C. The kitchen will fill with a gentle, savoury steam. Remove the chicken and, once cool enough to handle, shred it with two forks into chunky, juicy pieces.

Blanch the broccoli: Bring a large pot of salted water to a rolling boil. Drop in the broccoli florets and cook for exactly 2 minutes. You want them to turn a vibrant, emerald green and become tender-crisp. Drain immediately and plunge them into a bowl of ice water to stop the cooking. This preserves their colour and texture. Drain again and set aside.

Start the sauce: In a large frying pan or skillet, heat the olive oil over a medium heat. Add the chopped onion and cook for 5-6 minutes, stirring occasionally, until it becomes soft and translucent, releasing a sweet, pungent aroma. Add the minced garlic and cook for another 30 seconds until fragrant – you'll smell it immediately.

Build the creamy base: Pour the chicken stock into the pan, scraping the bottom with a wooden spoon to lift any browned bits – that's where the flavour lives. Let it simmer for 2 minutes until it reduces slightly. Lower the heat and stir in the double cream, Dijon mustard, and dried thyme. The sauce will look pale and loose now, but it will thicken as it bakes. Season generously with salt and pepper.

Assemble the dish: Preheat your oven to 190°C (170°C fan). Arrange the blanched broccoli in a single layer in a 20x30cm baking dish. Scatter the shredded chicken evenly over the top. Pour the creamy sauce over everything, making sure it settles into all the nooks and crannies. The dish should look glossy and inviting.

The golden finish: In a small bowl, mix the grated Cheddar with the breadcrumbs (if using). Sprinkle this mixture evenly over the casserole. Dot the top with small pieces of the butter – this will help the topping become crisp and golden. Bake for 20-25 minutes, until the sauce is bubbling up around the edges and the top is a deep, spotty gold. The smell at this point is utterly intoxicating – warm, cheesy, and comforting.

Rest and serve: Remove from the oven and let it stand for 5-10 minutes. This allows the sauce to settle and thicken further, so it doesn't run all over your plate. Serve it steaming hot, with the crust crackling softly as you spoon it out.