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Heat ½ tbsp oil in a 20cm-wide skillet or ovenproof frying pan, and fry the spring onions and courgettes for 5 mins until lightly golden. Stir in the frozen peas and sugar snap peas, and cook for a few minutes until the peas have defrosted. Tip in the butter beans, crème fraîche and 50ml water. Stir, then bubble for 2-3 mins until the sauce coats the greens. Transfer to a bowl, then stir in the mint and lemon zest. Season.
Clean the pan and heat the oven to 200C/180C fan/gas 6. Use 1 tbsp oil to brush the filo sheets all over, then stack them up. Add the filo stack to the pan, pressing it into the base and leaving plenty of overhang. You can cut the filo sheets in half, then use them to line the pan, if you find that easier.
Mix the egg into the greens and crème fraîche mixture, then spoon over the stack of filo. Scatter over the feta, then scrunch up the overhanging filo around the sides to make a crust. Brush the exposed filo with the remaining oil.
Return the pan to the heat and cook for 5 mins to crisp the filo base. Transfer to the oven and bake for 20-25 mins until the filo is fully crisp and golden, and the filling has set. Cut into wedges and serve with some salad leaves, if you like.