Heat the oil in a large, deep, heavy-based pan over medium heat. Add the onion and leek and cook for 5–6 minutes until softened.

Stir in the garlic and cook for 30 seconds until fragrant.

Increase the heat to medium–high and add the chicken, salt and pepper. Cook for 4–5 minutes, stirring occasionally, until the chicken is sealed.

Add the butter and let it melt for 30 seconds. Stir in the flour and cook for 1 minute, coating the chicken and vegetables to form a paste.

Gradually pour in half the chicken stock, whisking well, then add the remaining stock and whisk until smooth.

Stir in the cream, mustard and thyme. Reduce the heat slightly and simmer for 5–6 minutes, stirring often, until the sauce becomes thick, glossy and silky.

Add the peas (if using) and parmesan and stir until the cheese has melted through. Taste and adjust the seasoning. Cool the filling for 1 hour in the fridge.

Preheat the oven to 200°C (400°F) (180°C/350°F fan-forced). Remove and discard the thyme sprigs. Spoon the cold filling into a large oven-safe dish.

Cut the puff pastry into small rectangles. Lay them over the top of the filling, slightly overlapping but not stacked too thickly.

Press the pastry down gently around the edge of the dish and crimp/squeeze the pastry against the rim to seal.

Brush the pastry generously with the egg yolk. Sprinkle with a few sea salt flakes. Bake for 30 minutes or until the pastry is puffed and deeply golden.

Rest for 10 minutes before serving. Scatter with fresh thyme leaves and serve with mashed potatoes and steamed greens.