Preheat your oven to 190°C (fan 170°C). While it heats, lightly grease a 23x33cm baking dish with a little butter or oil — you’ll smell the warm metal as the oven comes up to temperature.

Cook the chicken. Heat the olive oil in a large frying pan over medium-high heat. Add the chicken chunks in a single layer — you should hear a satisfying sizzle. Cook for 5-6 minutes, turning once, until the chicken is golden brown on the outside and no longer pink inside. The edges will look crisp and the pieces will feel firm to the touch. Transfer the chicken to a plate and set aside.

Make the sauce base. In the same pan (don’t wipe it clean — those browned bits are flavour), add the chopped onion. Cook for 3-4 minutes until it turns translucent and smells sweet. Add the minced garlic and cook for another 30 seconds until fragrant. Sprinkle the flour over the onion mixture and stir constantly for 1 minute — the paste will turn a pale gold and smell slightly nutty.

Build the creamy sauce. Slowly pour in the chicken stock while whisking continuously to prevent lumps. The mixture will thicken noticeably after about 2 minutes, turning from a thin liquid into a glossy sauce that coats the back of a spoon. Stir in the double cream, Dijon mustard, salt, and pepper. Let it simmer gently for 2 minutes — you’ll see small bubbles breaking the surface and the sauce will smell rich and savoury.

Add the broccoli and cheese. Tip the broccoli florets into the sauce and stir well. Cook for 2 minutes until the broccoli turns bright green and starts to soften slightly. Remove the pan from the heat and stir in 100g of the grated cheddar and all the Parmesan. The cheese will melt into the sauce, creating a smooth, creamy mixture with a pale yellow colour.

Combine and assemble. Return the cooked chicken to the pan and stir everything together until evenly coated. Pour the entire mixture into your prepared baking dish and spread it into an even layer. The sauce should look thick and glossy, with chunks of chicken and broccoli visible throughout.

Top and bake. Sprinkle the remaining 50g of cheddar over the top, followed by the breadcrumbs if using. Bake for 18-20 minutes until the top is golden brown and bubbling around the edges — you’ll hear the sauce gently bubbling and the kitchen will fill with the smell of melted cheese. The breadcrumbs should look crisp and toasted.

Rest before serving. Remove the dish from the oven and let it stand for 5 minutes. This allows the sauce to set slightly so it doesn’t run all over the plate. The surface will still be hot but the casserole will hold its shape when spooned out.