First, make the pastry. Coarsely grate the butter onto a cold plate (do not handle the grated butter). Return to the freezer or put into the fridge to keep cold.
Sift both types of flour into a large bowl and mix in the dried sage, a pinch of salt and some pepper. Without handling the butter, drop it into the flour mix and, using a blunt knife, mix the grated butter into the flour to coat it all evenly. Sprinkle over two tablespoons of ice-cold water and mix in with the knife, then gradually add a further two to three tablespoons of water until the mix starts to come together.
Finish mixing to a pastry dough with your hands but do not handle the pastry too much. If you need more water, add it a couple of drops at a time. The pastry should leave the sides of the bowl clean.
Flatten the pastry into a square shape and wrap it in cling film. Leave to rest in the fridge for 30–45 minutes.
Meanwhile, make the filling. Put all of the filling ingredients, except the black pudding, into a large bowl, with some salt and pepper. Start mixing with a large spoon – then I like to get my hands in and squish it all together. Fry off a small piece of the filling in a small pan to check the seasoning. Add more to taste. Cover and set aside until needed.
Remove the black outer coating from the black pudding and cut the pudding into 1cm sticks.
Set the chilled pastry on a lightly floured surface and roll out into a 60 x 30cm rectangle that’s about 3mm thick. Turn the pastry as you roll to stop it from springing back. Again, try not to handle the pastry too much.
Trim the edges of the pastry rectangle so they are straight and neat. Spoon the sausagemeat filling into the centre of the pastry and squeeze it together to make a sausage shape that runs across the pastry rectangle, parallel to the 30cm sides (I find flouring your hands helps to do this). Push the sticks of black pudding into the filling in straight lines from end to end.
With cold, floured hands, lift one 30cm side of the pastry up to the middle over the sausage filling and brush the outside edge with beaten egg (or milk). Lift the opposite side of the pastry up to slightly overlap and gently squeeze the edges together to create a sealed join of pastry. Roll the whole thing over so the join is on the bottom. Lift on to a tray and place in the freezer to chill for 10 minutes.
Remove from the freezer and, using a sharp knife, cut across into 5cm pieces. Arrange these, with the join underneath, on a greased baking sheet that has been lined with greaseproof paper. Return to the freezer to keep cold while you preheat the oven to 180°C fan (200°C/400°F/Gas Mark 6).
Glaze the pastry with beaten egg (or milk) and snip a small ‘V’ on top of each roll. Bake for 20–30 minutes until the pastry is golden brown and the sausage filling is cooked and no longer pink.
Remove from the oven and gently lift the sausage rolls from the baking sheet on to a wire rack to cool.