Fry the diced onion in the sunflower oil until soft in a shallow pan that has a lid. Add diced turkey and fry until browned. Add the creme fraiche with the concentrated jelly stock. Stir until stock has dissolved, then add the tarragon and season to taste.
Stir and cover until turkey is cooked through, about 10 to 15 minutes.
Preheat the oven to 180 C / Gas 4. Cool mixture and turn into pie dish.
Roll out defrosted puff pastry and cover pie dish. Trim, if necessary. Glaze pastry with beaten egg.
Bake in the pre-heated oven for 25 to 30 minutes, or until the pastry is golden brown.