Preheat your oven to 190°C (170°C fan/Gas Mark 5). Drizzle 1 tablespoon of olive oil into a large, oven-safe casserole dish or a 9x13 inch baking dish. Add the cut chicken pieces to the dish, ensuring they are spread in a single layer. You'll notice the raw chicken has a pale pink colour.

In a large bowl, whisk together the plain flour and 60ml of the chicken stock until smooth, creating a slurry. This mixture will be pale and slightly thick. Set aside.

Heat the remaining 1 tablespoon of olive oil in a large, deep frying pan or skillet over medium heat. Add the chopped onion, carrots, and celery. Sauté for 6-8 minutes, stirring occasionally, until the vegetables begin to soften and the onion turns translucent, releasing a sweet aroma.

Add the minced garlic, sliced mushrooms, dried thyme, and crushed rosemary to the pan. Continue to cook for another 3-4 minutes, stirring frequently, until the garlic becomes fragrant and the mushrooms have softened and released their moisture, developing a lovely earthy smell.

Pour the remaining 640ml of chicken stock and the semi-skimmed milk into the pan with the vegetables. Bring the mixture to a gentle simmer, stirring well. The liquid will warm and release steam.

Gradually whisk in the flour and stock slurry you prepared earlier. Continue to stir and simmer for 2-3 minutes until the sauce thickens slightly, achieving a light, creamy consistency that coats the back of a spoon. Season with salt and freshly ground black pepper to your taste.

Pour this creamy vegetable sauce evenly over the chicken in the prepared baking dish. Scatter the rinsed long-grain white rice over the sauce, ensuring it's as evenly distributed as possible. You'll see the rice grains nestled amongst the chicken and sauce.

Cover the baking dish tightly with aluminium foil. Bake for 30 minutes. When you carefully remove the foil, you should see the rice has absorbed most of the liquid and the chicken is turning opaque and white.

Remove the foil, stir in the frozen peas, and gently combine everything in the dish. Return the uncovered dish to the oven and bake for another 15-20 minutes, or until the rice is fully cooked and fluffy, the chicken is cooked through (reaching an internal temperature of 74°C), and the top is lightly golden brown. The casserole will have a bubbling sound as it finishes.

Once baked, remove the casserole from the oven and let it rest for 5-10 minutes. This resting period allows the flavours to meld and the rice to fully set. Garnish generously with fresh chopped parsley before serving. The vibrant green parsley will contrast beautifully with the golden casserole.