Pre-heat the BBQ for 2-3 minutes with the paella pan in place.
Add a dash of oil to the paella pan and add the chicken, cook over a low-medium heat for 3-4 minutes, try and refrain from stirring/moving the chicken around and instead allow it to seal.
Then add the leeks and cook for a few minutes until they begin to soften. Add garlic, mustard and stock, cook for 10 minutes or until the stock has reduced a little. Then add the cream cheese, or you can use single cream / crème fraiche. Season to taste and allow it to bubble up a little.
Lay the puff pastry sheet over the top and trim to size. Brush with beaten egg and then pop the lid on the BBQ and cook for 30-35 minutes or until the top is golden brown.