Fry the chicken fillet in a little olive oil on medium heat until just cooked and allow to cool before slicing thinly. Turn the heat to low and in the same pan gently fry the sliced leeks in the butter until soft and translucent. Season with pepper and remove from the heat. Return the chicken to the pan and toss through the wholegrain mustard and some chopped oregano leaves.
Preheat the oven to 180°C. Using a non stick 30cm rectangular tart pan line the pastry in the tart pan. Prick the pastry with a fork and bake in the oven for 10 mins before removing to a wire rack. In a small bowl whisk together the eggs, sour cream and cream.
Fill the tart base with the chicken and leek filling and crumble the fetta cheese over the top. Pour in enough of the egg mixture to fill the tart to the top and bake for 30 mins.