Preheat oven to 200°C and line a large baking tray with baking paper.
In a large fry pan, heat butter and oil. Sauté onions gently for 10 mins. And then the garlic for 2 mins.
Add the mushrooms and cook for a further 5-8 mins until soft.
Stir in the crème fraiche and lemon zest then simmer.
Stir in the cornflour water until the sauce thickens (about 5 mins).
Add the chicken and simmer for a further 5 mins. Season to taste.
Cut the puff pastry into 15cm rounds and egg wash around the edges.
Put a spoonful of mixture into the centre of each pastry round and spread along the middle, and sprinkle with thyme.
Fold pastry over to form a crescent and crimp the edges to seal. Repeat until all the mixture’s gone.
Egg wash the turnovers, sprinkle with sesame seeds then bake for 12-15 mins, or until golden and flaky.