Preheat oven to 180°C fan-forced. Salt and pepper the chicken thighs.
Heat oil in a large casserole or oven-safe pan. Brown the chicken on both sides, 4 minutes per side. Set aside.
In the same pan, cook leeks and onion 15 minutes until soft and slightly golden. Add garlic, stir 30 seconds.
Sprinkle in flour. Stir 1 minute. Add wine, scraping the bottom. Reduce by half, 3 minutes.
Add stock, cream and tarragon. Return chicken thighs to the pan, nestled into the sauce.
Roll the pastry sheet to fit the top of the dish. Lay it over. Brush with beaten egg. Cut 4 small slits in the top to vent steam.
Bake 35-40 minutes until pastry is deep golden and the sauce is bubbling at the edges.
Rest 5 minutes before serving.