For the filling
Preheat the oven to 200°C/400°F/Gas mark 6.
Melt the butter in a frying pan, and sauté the leek and garlic, stirring frequently, for 2 minutes. Add the peppers, mushrooms and asparagus, and cook for 3–4 minutes.
Add the flour and fry for 1 minute. Remove the pan from the heat, and stir in the vegetable stock, milk and white wine. Return the pan to the heat and bring to the boil, stirring, until thickened. Stir in the oregano, and season with salt and pepper.
Roll out half of the pastry on a lightly floured surface to form a rectangle measuring 38 x 15cm/15 x 6in. Roll out the other half of the pastry to the same shape, but a little larger all round. Put the smaller rectangle o a baking tray lined with dampened baking parchment.
Spoon the filling evenly on top of the smaller rectangle, leaving a 1cm/1⁄2in margin around the edges.
Using a sharp knife, cut parallel diagonal slits across the larger rectangle to within 2cm/1in on each of the long edges.
Brush the edges of the smaller rectangle with beaten egg, and place the larger rectangle on top, pressing the edges to seal.
Brush the whole jalousie with egg to glaze, and bake in the oven for 30–35 minutes until risen and golden. Serve immediately