Preheat the oven to gas 5, 190°C, fan 170°C. Lightly grease a 9-inch round tin.
On a lightly floured work surface, unroll 1 pack of pastry and roll out a little larger than the tin so that it lines the base and comes three quarters of the way up the side of the cake tin. Press into the corners of the tin and up against the sides, easing out any pleats of pastry. Chill while making the filling.
Using kitchen scissors, snip the bacon into small pieces. Heat the butter in a frying pan and fry for 3-4 minutes until cooked. Add the onions and cook until transparent, but not browned. Tip this into a mixing bowl. Coarsely grate the unpeeled apples into the onion mixture (discarding the cores) and stir in the herbs. Add the sausagemeat, and using your hands, squeeze the mixture together well. Place inside the pastry case.
Unroll the second sheet of pastry and cut to fit the top of the pie. Put in place and press around the edge to seal. Trim the edge.
Re-roll the pastry trimmings and cut out letters to decorate the top. Brush with the beaten egg and bake for 1 hour until golden brown. Serve the pie warm or, better still, cold.
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