Makes 5 empanadas
Clean and de-beard the mussels. Discard any that remain open if tapped. Put them to one side. Peel and finely chop the onions and garlic.
In a pan big enough to fit all the mussels, melt the butter over a medium heat. Add the onions and garlic, cooking them until totally soft but not coloured, about 10 minutes. When they are done, add the mussels, put the lid on the pan and cook for 3-4 minutes. You want the mussels to be just done and very soft. Remove the mussels, making sure as much of their juice remains in the pan as possible. Discard any that fail to open. Stir in the saffron and the cumin, leaving the juices gently simmering over the heat.
Put the mussels to cool. In another pan, hard-boil one of the eggs for 7 minutes before allowing it to cool as well. Pick out all the mussel meat and discard the shells. Reduce the mussel juice by half, take it off the heat and allow it to cool also.
Roughly chop the mussel meat, and put it in a bowl before grating in the hard-boiled egg on the largest hole setting. Pick, wash, dry and roughly chop the parsley, adding this to the mussel stuffing. Mix in the remaining raw egg yolk. Give the stuffing a good grind of black pepper and about 1 teaspoon salt. Mix everything well before covering the bowl and leaving it in the fridge.
To make the pastry, sift the flour into a large mixing bowl and add the salt. Chop the butter, which should be cold, into board game-dice pieces and drop them in the bowl. Pinch and rub the butter through the flour until the little lumps of butter are blended in to make a fine crumb feel. Add the water, stirring round the inside of the bowl with a table knife until the dough comes together. Lift it out and knead it on a flat floured surface for 2 minutes until it becomes smooth. Wrap it in clingfilm and leave in the fridge to rest for half an hour at least.
Comes the time to make the empanadas, beat the egg with a little cold water. Working on a floured surface, roll the pastry out to about the thickness of a 20 pence piece, and then cut into five circles the size of a 7-inch single (if you don't know what one of these is, ask your parents). Place 2 good tablespoons of the filling in the centre of each circle. Dipping your finger in the egg/water mix, wipe it around half the circumference of your pastry circles. Pull the opposite side over on to the rim of the surface you have just wetted. Roll the edges in twice and pinch from end to end. Repeat this procedure until all the filling is used.
Get a large saucepan and fill with about 5cm vegetable oil, or turn your fryer on to 180ºC/350ºF. When the oil has come to heat it must not be smoking, as danger in your kitchen will be impending, test the temperature by plopping in a little coin-sized piece of pinched-out pastry: it should whiz around, fizzling madly. Do not put the empanadas in until the oil is ready, otherwise they will sink and become uneatably greasy.
Pick the empanadas up carefully, as you will find they are quite fragile. Pop them into the hot oil, two at a time, and cook them on both sides until you achieve a dark biscuit brown. Remove them to waiting kitchen paper, using a slotted spoon. Please eat while they are hot. A good splash of hot chilli sauce suits them well, as does a very cold bottle of lager.