Oven-Baked Chicken Divan

Prepare the broccoli by steaming the florets until they are just tender-crisp, ensuring they retain a slight bite and bright green color. Immediately plunge them into a bowl of ice-cold water to stop the cooking process, then drain thoroughly and set aside. In a medium bowl, toss the diced chicken breast with half of the plain flour, then season generously with sea salt flakes and freshly ground black pepper, ensuring each piece is lightly coated.

In a large frying pan or skillet, melt half of the unsalted butter with the extra virgin olive oil over medium heat. Once the butter is shimmering, add the floured chicken in batches, taking care not to overcrowd the pan. Cook until the chicken is beautifully browned on all sides and mostly cooked through, about 5-7 minutes per batch. Transfer the browned chicken to a large mixing bowl, adding the drained, blanched broccoli.

In a medium saucepan, melt the remaining unsalted butter over medium heat. Add the very finely diced brown onion and sliced mushrooms, along with the ground nutmeg. Sauté for 5-7 minutes, stirring occasionally, until the vegetables are well softened and the mushrooms have released their moisture. Stir in the remaining plain flour and cook for another minute, continuously stirring, to create a smooth roux. Gradually whisk in the full-fat milk, ensuring no lumps form, and bring the mixture to a gentle simmer, allowing it to thicken into a creamy sauce.

Fold in two cups of the grated vintage cheddar cheese, the Dijon mustard, and the Worcestershire sauce until the cheese is fully melted and the sauce is smooth. Stir in the sour cream until thoroughly combined, making the sauce luxurious. Taste and adjust seasoning with additional sea salt flakes and freshly ground black pepper as desired. Pour this rich, creamy sauce over the browned chicken and broccoli in the large mixing bowl and gently mix until all ingredients are evenly coated.

Spoon the entire chicken and broccoli mixture evenly into a 2.5-liter (approximately 10-cup) baking dish. In a small bowl, combine the remaining one cup of grated vintage cheddar cheese with the multigrain breadcrumbs. Sprinkle this cheesy breadcrumb mixture generously over the top of the casserole, ensuring an even layer for a delightful crispy crust.

Place the baking dish into the preheated oven and bake for 25 minutes, or until the topping is a deep golden brown and the filling is bubbling hot around the edges. This indicates the sauce has warmed through and the flavors have melded.

Allow it to rest for a few minutes before garnishing generously with freshly picked flat-leaf parsley leaves for a vibrant finish. Serve immediately with a crisp mixed green salad to balance the richness of the dish.