Prepare the vegetables: Sauté the onion and leek in olive oil. Add the diced red pepper and continue to sauté.
Cook the mushrooms: Add the mushrooms to the pan and cook until they're soft. Season with salt, pepper, oregano, and thyme.
Add tomato paste and breadcrumbs: Stir in the tomato paste and grated breadcrumbs to the mushroom mixture to help bind the filling.
Add cheese and olives: Off the heat, mix in the chopped parsley, sliced black olives, and grated yellow cheese.
Prepare the pastry: In an oiled baking pan, lay four sheets of the rustic golden pastry on the bottom, brushing each with olive oil.
Add the filling: Spread the mushroom filling evenly over the pastry.
Cover with pastry: Cover the filling with three more pastry sheets, brushing each with olive oil.
Bake: Carve the top of the pie to allow steam to escape. Bake in a preheated oven at 170°C for about an hour or until golden and crispy.