In a pan over medium-high heat, add half the coconut milk and the curry paste. Fry for 1–2 mins until fragrant and the red oil starts to separate. Stir in the flour, then add the remaining coconut milk and chicken stock to create a rich, smooth sauce.
Add the chicken and simmer for 5 mins. Then add capsicum, lime leaves, sugar, and fish sauce.
Continue simmering until the chicken is cooked through and the sauce has thickened slightly. Turn off the heat and stir in Thai basil, chilli, and lime juice. Taste and adjust seasoning. Set aside to cool.
Preheat oven to 180°C. Line a baking dish with shortcrust pastry, pressing it up the sides. Trim any excess. Line with baking paper and fill with rice or baking weights. Bake for 25 mins, removing the paper and weights in the last 5 mins.
Once the base is baked, fill with the cooled curry mixture. Brush the pastry edges with egg wash, then cover with puff pastry. Press the edges with a fork to seal, cut a small cross in the centre to let steam escape, and brush the top with egg wash.
Bake at 180°C for 30–40 mins until the top is golden and puffed. Let cool slightly, then dig in!
