Preheat the oven to 220°C (200°C fan-forced) and line a baking tray with baking paper.
Place lentils in a small saucepan covered with water over a medium heat. Bring to the boil and simmer for 15 minutes or until tender. Drain and set aside until required.
For the filling, heat butter and olive oil together in a sauté pan over a medium-low heat, until butter melts. Add garlic, chili and mushrooms stirring until well coated. Cook uncovered for about 10 minutes or until mushrooms are cooked. Add spinach, cover pan and cook for 2 minutes or until spinach wilts then add lemon juice, olives and flat leaf parsley. Remove from heat, add cooked lentils and season with salt and pepper, stirring gently until combined. Set aside and allow mixture to cool completely, you can speed this up by spreading out on a tray.
Place puff pastry sheet on a lightly floured work surface and halve lengthways, to create two equal sized rectangles. Roll each rectangle to approximately 23cm x 35cm. Place each piece of pastry on a baking tray lined with baking paper.
Evenly divide mixture into two and place in a mound on ‘one half’ of each piece of pastry, allowing a 2cm wide border around the edge, top the with the mozzarella. Brush the border with egg wash. Bring the other pastry half over the filling to enclose it like a pillow. Use a fork to help crimp the edges and seal the filling in properly. Brush all over with the beaten egg. Using a very sharp knife score the top of the pie (if pastry too soft pop in the freezer for 10 minutes to chill).
Bake in preheated oven for 10 minutes, then reduce oven temperature to 200°C (180°C fan-forced) and cook for a further 20 minutes or until pastry is puffed, golden and cooked through
Note: We used Scamorza (which is similar to mozzarella)