First, prepare the chicken. Heat the olive oil in a large, heavy-based saucepan or Dutch oven over a medium-high heat. Season the chicken pieces with salt and pepper, then add them to the pan in a single layer (you may need to do this in two batches). Sear for 3-4 minutes, until lightly golden on all sides. The chicken doesn't need to be cooked through at this stage. Remove it with a slotted spoon and set aside.

Reduce the heat to medium and add the butter to the same pan. Once melted, add the chopped onion and cook for 5-7 minutes, stirring occasionally, until soft and translucent. Add the sliced mushrooms and cook for another 5 minutes until they have released their liquid and started to brown. Stir in the minced garlic and cook for one minute more until fragrant.

Sprinkle the plain flour over the vegetables and stir continuously for 2 minutes. This step cooks out the raw flour taste and is crucial for a thick, glossy sauce.

If using, pour in the white wine to deglaze the pan, scraping up any browned bits from the bottom. Let it bubble away and reduce by about half. Gradually pour in the hot chicken stock, stirring constantly to avoid lumps. Once all the stock is incorporated, bring the mixture to a gentle simmer.

Pour in the double cream, then add the fresh thyme leaves and Dijon mustard. Return the seared chicken to the pan along with any resting juices. Season with salt and pepper to your taste.

Let the filling simmer gently for 10-12 minutes, stirring occasionally, until the sauce has thickened enough to coat the back of a spoon and the chicken is cooked through. Stir in the frozen peas and cook for one final minute. Remove from the heat and allow the filling to cool for at least 20 minutes. What works best for me is transferring it to a shallow dish to speed up the cooling process.

Preheat your oven to 200°C (180°C fan). Spoon the cooled chicken filling into a 1.5-litre pie dish.

Unroll your sheet of puff pastry and lay it over the pie dish. Use a sharp knife to trim the excess pastry around the rim. Crimp the edges with a fork or your fingers to seal. You can use the offcuts to decorate the top with leaves or other shapes if you like.

Brush the top of the pastry evenly with the beaten egg. This will give it a beautiful, glossy, golden-brown finish. Cut a couple of small slits in the centre of the lid to allow steam to escape during baking.

Place the pie on a baking tray (to catch any potential spills) and bake for 25-30 minutes, or until the pastry is puffed up, deeply golden, and the filling is bubbling hot. Let it rest for 5-10 minutes before serving.