Preheat and Prep: Preheat your oven to 190°C (170°C fan). Lightly grease a 9x13 inch baking dish. The warmth of the oven will soon start to fill your kitchen with a gentle heat.

Brown the Chicken: Heat the olive oil in a large frying pan over a medium-high heat. Add the chicken chunks in a single layer. Let them sizzle undisturbed for 4-5 minutes until the underside is a deep, golden brown. The sound of the searing chicken is the first promise of good things to come. Turn and cook for another 2-3 minutes until just cooked through. Remove the chicken from the pan and set aside.

Soften the Leeks: Reduce the heat to medium. Add the sliced leek to the same pan. Stir it gently, scraping up any browned bits from the bottom of the pan. Cook for 5-6 minutes, until the leek is soft and translucent. The aroma will be sweet and mellow. Add the minced garlic and cook for one more minute until fragrant.

Create the Sauce: Pour the chicken stock into the pan. Bring it to a gentle simmer, and let it bubble for 2 minutes, allowing it to reduce slightly. The stock will take on a lovely, savoury depth. Whisk in the double cream, Dijon mustard, and dried thyme. Continue to simmer for 3-4 minutes, stirring occasionally. The sauce will thicken enough to coat the back of a spoon, and its colour will be a pale, inviting gold. Season generously with salt and pepper.

Blanch the Broccoli: While the sauce simmers, bring a small pot of salted water to a rolling boil. Add the broccoli florets and blanch for 2 minutes. They should turn a brilliant, vibrant green. Drain immediately and plunge them into a bowl of cold water to stop the cooking process. This step ensures your broccoli stays perfectly tender, not mushy.

Assemble the Casserole: Spread the cooked chicken and blanched broccoli evenly in the prepared baking dish. Pour the creamy leek sauce over the top, ensuring everything is nicely coated. The dish will look wonderfully rustic and full of promise.

Top and Bake: In a small bowl, mix the grated cheddar cheese with the fresh breadcrumbs. Sprinkle this mixture evenly over the casserole. Bake for 20-25 minutes, or until the top is golden brown and bubbling. You'll hear the gentle pop and hiss of the sauce as it bakes, and the smell of melted cheese and toasted breadcrumbs will be irresistible.

Rest and Serve: Once baked, remove the casserole from the oven and let it rest for 5-10 minutes. This is a crucial step that allows the sauce to set slightly, making it easier to serve. The final dish will be a beautiful landscape of golden crust, creamy sauce, and vibrant green flecks of broccoli.