Preheat oven to 200°C fan-forced. Heat 20g butter in a large deep frying pan over high heat. Add the leeks and mushrooms and cook for 3-5 minutes or until browned. Add garlic and cook, stirring for 1 minute. Remove from the heat and stir in spinach leaves, until just wilted. Set aside to cool slightly.
Whisk eggs and cream in a medium size bowl, Add mushroom mixture, cheese and lemon rind. Season with salt and pepper.
Lay a 50cm long piece of baking paper on the bench. Top with 1 layer of filo pastry, brush with melted butter, top with another sheet of filo pastry and repeat until all pastry sheets are layered on top of each other.
Heat an oiled large deep frying pan with oven-proof handle over medium heat. Lift baking paper sheet into frying pan, easing down the edges into the pan. Stir egg and mushroom mixture and pour into pastry case.
Scrunch the pastry edges over the mushroom mixture. Transfer to the oven and cook for 20-25 minutes or until the egg is set and the pastry is golden.
Serve with lemon wedges and freshly ground black pepper.