Heat 1 tbsp olive oil with a knob of the butter in pan and cook the onion until golden. Remove to a bowl.
In the same pan, cook three quarters of the courgettes with all the spring onions until the mixture has collapsed into a purée and is no longer wet. The idea is to drive out as much of the excess moisture as possible. Transfer to the onion bowl.
Heat the rest of the olive oil and another knob of butter, then fry the rest of the courgettes. These need to be cooked until they have exuded a lot of water, but don’t take them to a purée – this gives the pie a bit of texture.
Add these to the bowl along with the feta, bulgur wheat and herbs. Season.
Melt the rest of the butter and use some of it to brush the base and sides of a 26cm springform cake tin.
Put four whole sheets of filo into the tin, positioning them like the spokes of a wheel and overlapping, all the way round. The sheets should hang over the sides of the tin by about 11cm. Butter each sheet as you go along. Do an overlapping second layer in the same way.
Spoon the courgette mixture on top of the filo. Bring the pastry up over the top of the pie, but don’t completely cover it – scrunch the ends of the filo all the way round. Brush with the remaining butter. Bake in the oven for 45-50 minutes, or until the pie is a lovely golden colour.