Mushroom Leek and Egg Tart

Preheat the oven to Gas Mark 5, 190oC (375oF) 

Beat one egg in a small bowl for the egg wash. 

To prepare the pastry: Lay the puff pastry out on a large baking sheet. Cut a 2 cm strip from the perimeter of the puff pastry from each of the four sides. Brush the remaining puff pastry with the egg wash, then prick all over with a fork to prevent puffing.

Place the pastry strips along the edges of all four sides to form a raised border, and gently press to seal. Trim the excess and brush the border with the remaining egg wash. Refrigerate the dough for 5 minutes. 

Transfer the baking sheet to the oven and bake until the pastry is puffed and golden. This will take approximately 10 minutes.  

While the pastry bakes, heat half the oil and butter in a frying pan until gently foaming. Cut the leek in half lengthways, then thinly slice and add to the frying pan. Cook over medium-high heat.

Stir and shake the pan now and then. Cook until the leeks soften and turn pale golden at the edges. Transfer to a bowl.

To cook the vegetables: Add the remaining oil and butter to the frying pan. Then add the mushrooms and cook for 4-5 minutes until they soften. Return the leeks to the pan, stir in the thyme, and season.

Mix the crème fraîche, mustard and lemon zest in a small bowl, then season with salt and pepper. Working within the border, spread this mixture over the pastry.  

Spoon the leek and mushroom mixture over the pastry's surface. Make 4 dips for the eggs, then crack the eggs into them.

Sprinkle over the cheese. Bake until the egg whites are firm, and the yolks are still runny. The puff pastry should be golden brown too. This will take about 15-17 minutes.

To serve: Sprinkle over some chives. Cut the tart into quarters and serve immediately with a green salad.