Preheat and Prepare: Preheat your oven to 200°C (180°C fan). Have a 2-litre ovenproof dish ready.

Sauté the Vegetables: In a large saucepan or Dutch oven, melt 50g of butter over a medium heat. Add the chopped onion, carrots, and celery. Cook for 8-10 minutes, stirring occasionally, until the vegetables have softened but not browned.

Make the Sauce: Stir the 50g of plain flour into the vegetables and cook for one minute – this gets rid of the raw flour taste. Gradually pour in the warm chicken stock, whisking continuously to avoid lumps. Bring the mixture to a simmer, and it will start to thicken.

Finish the Filling: Once thickened, stir in the double cream, cooked chicken, frozen peas, and fresh thyme leaves. Season generously with salt and black pepper. Let it simmer gently for 2-3 minutes. Pour the filling into your ovenproof dish and set aside.

Make the Biscuit Dough: In a medium bowl, whisk together the 250g plain flour, baking powder, and salt. Add the cold, cubed butter. Using your fingertips, rub the butter into the flour until the mixture resembles coarse breadcrumbs. I find that working quickly here is key to keeping the butter cold.

Form the Biscuits: Pour in the buttermilk and use a fork to gently mix until a soft, slightly sticky dough forms. Be careful not to overmix; just bring it together.

Assemble the Pie: Drop spoonfuls of the biscuit dough over the hot chicken filling, leaving a little space between each one as they will expand. You should get about 8-10 rustic-looking biscuits on top.

Bake to Perfection: Brush the tops of the biscuits lightly with milk. Place the dish on a baking tray (to catch any potential drips) and bake for 25-30 minutes, or until the filling is bubbling and the biscuits are tall, golden brown, and cooked through. Let it rest for 5 minutes before serving.