Chop the onion finely and soften it in a little butter or oil, then fry hard until frazzled. Remove and reserve. Dice the mushrooms and fry them in the fat remaining in the pan; stir and turn them as necessary to drive off most of their moisture and to concentrate flavour. Mix the two vegetables together and season with plenty of salt and pepper. Stir in the cumin, coriander and grated cheese. When cool bind with the beaten eggs.
Unwrap the pastry one sheet at a time. Keep the rest covered with a damp cloth to prevent drying out. Cut each sheet into strips about 3 x 10 inches long and brush on one side only with melted butter. Put one rounded metal teaspoonful of the mixture near the short edge of the buttery side of the first pastry strip - about 1 inch from the bottom and slightly to the left-hand side. Fold the bottom right-hand corner of the pastry diagonally over the filling to make a triangle. Continue folding the pastry at right angles up the whole lenghth of the strip so you end up with a neat little triangular parcel. Make more little pies in the same way until all the filling is used up.
To cook, simply brush the pies all over with melted butter or oil, arrange them side by side on baking trays and bake at 350-375 F (180-190 C) gas mark 4/5 for about 20 minutes, until the pastry is golden and crisp. It is best to turn the pies over after the first 10 minutes and brush with more butter or oil. For even crisper results the pies can be deep-fried.