Prepare the Pasta: Preheat your oven to 190°C (170°C fan/Gas Mark 5). Bring a large pot of salted water to a rolling boil. Add the pasta and cook according to package instructions until it’s al dente – this means it should be tender but still have a slight bite when you test a piece. You'll notice the pasta swelling and becoming pliable. Drain thoroughly in a colander and set aside.

Sauté the Aromatics and Chicken: While the pasta cooks, heat the olive oil in a large frying pan over a medium heat. Add the chopped onion and cook for 3-4 minutes until it softens and turns translucent, releasing a sweet aroma. Stir in the minced garlic and cook for another minute until fragrant, making sure it doesn’t brown. Add the cubed chicken breast and cook for 5-7 minutes, stirring occasionally, until it's sealed and lightly golden on all sides, losing its raw pink colour.

Add Vegetables: Introduce the chopped red pepper and sliced mushrooms to the pan with the chicken and onions. Continue to cook for 5-7 minutes, stirring occasionally. The vegetables will soften slightly and release some of their moisture, and the mushrooms will take on a lovely light brown hue.

Create the Boursin Sauce: Reduce the heat to low. Crumble the Boursin cheese into the pan, stirring gently until it begins to melt and coat the chicken and vegetables. Pour in the milk and chicken stock, then sprinkle in the dried oregano and black pepper. Stir continuously until the Boursin has completely melted and the sauce looks smooth and creamy, gently simmering. You'll notice a delightful herby fragrance filling your kitchen. Taste and adjust for salt if needed, remembering the stock and Boursin already contain salt.

Combine and Bake: Add the drained pasta to the pan with the creamy Boursin sauce, tossing thoroughly with tongs or a pasta fork until every strand is beautifully coated. Transfer the entire mixture into a large ovenproof dish. Sprinkle the grated cheddar cheese evenly over the top. The dish will look rich and inviting, ready for its golden transformation.

Bake to Perfection: Place the ovenproof dish into the preheated oven and bake for 20-25 minutes. You'll know it's ready when the cheese on top is melted, bubbly, and a beautiful golden-brown colour, and the sauce is gently bubbling around the edges. Carefully remove from the oven and let it stand for 5 minutes before serving. Garnish with fresh chopped parsley if desired.