Step 1

Preheat the oven to 220C/200C Fan/Gas Toss the carrots and parsnips with 2 tablespoons of oil, a pinch of salt and lots of ground black pepper in a large bowl then scatter over a baking tray. Bake for 30–40 minutes, turning after 20 minutes, or until tender and lightly browned. Don’t worry if the oven hasn’t reached temperature when you add the vegetables.

Step 2

Meanwhile make the pie filling, heat 1 tablespoon of the oil in a large frying pan, add half of the mushrooms and season with ground black pepper. Fry over a high heat for 2–3 minutes or until lightly browned, stirring regularly. Tip into a bowl. Add a tablespoon more oil and the remaining mushrooms to the pan and cook for 2–3 minutes more. Add to the first batch.

Step 3

Add a tablespoon of oil to the same frying pan and fry the onion over high heat for 3–4 minutes, or until softened and lightly browned. Reduce the heat a little, stir in the flour and cook for a few seconds, stirring constantly.

Step 4

Gradually add the stock, just a little at a time, stirring well between each addition. Stir in the milk and bring to a gentle simmer, stirring constantly. Remove from the heat and stir in the cooked mushrooms. Adjust the seasoning to taste. Tip the mushroom mixture into a 1 litre pie dish and place on a baking tray.

Step 5

Unroll the sheet of pastry and score in a criss-cross pattern with the tip of a knife without pressing all the way through. Put the pastry on top of the pie dish and squeeze around the sides to seal.

Step 6

Trim off any excess with kitchen scissors, leaving a border around the edge of around 2cm/¾in as the pastry will shrink a little as it cooks. Work quickly before the heat from the filling softens the pastry.

Step 7

Bake the pie on a baking tray (to catch any drips) on the shelf above the vegetables for 20 minutes, or until puffed up and pale golden brown. Cook the peas according to the pack instructions and serve alongside the pie and roasted vegetables.