Place chicken thighs, quartered onion, halved carrots, bay leaf and water in a large saucepan. Bring to a gentle simmer over medium-high heat.

Reduce heat to low and poach, uncovered, for 12–15 minutes until the thickest part of the thigh reaches 75°C on a meat thermometer or juices run clear when pierced.

Remove chicken to a plate and let cool slightly. Strain poaching liquid through a sieve into a bowl and reserve 200ml; discard vegetables.

Shred the cooled chicken into bite-sized pieces and set aside.

Preheat oven to 200°C fan-forced.

Wash leeks thoroughly and slice into 1.5cm rounds. Melt butter in the same saucepan over medium heat.

Add leeks and cook, stirring occasionally, for 6–8 minutes until softened and lightly golden at the edges.

Stir in minced garlic and cook for 1 minute until fragrant.

Sprinkle flour over the leeks and stir constantly for 2 minutes to cook out the raw taste.

Pour in cream and reserved poaching liquid slowly, stirring continuously to avoid lumps. Bring to a gentle simmer.

Stir in mustard, thyme, salt, pepper and nutmeg. Add shredded chicken and stir through. Taste and adjust seasoning.

Transfer filling to a 2-litre baking dish in an even layer and let cool for 5 minutes.

Lay puff pastry over the filling, allowing it to cover the entire surface but not overhang the dish. Trim excess with a sharp knife if needed.

Brush pastry evenly with beaten egg and sprinkle with sea salt flakes.

Bake on the middle shelf for 18–22 minutes until the pastry is puffed and deeply golden brown.

Rest for 5 minutes before serving. The filling will set slightly as it cools.