Preheat the oven to gas 7, 220ºC, fan 200ºC. Unroll the pastry on a big chopping board and cut in half lengthways.
Squeeze the sausages out of their skins into a bowl, add the sage and mix together well. Divide into two and shape into two long thin sausages the length of the pastry. Put one long thin sausage along each pastry strip, just to the left of the middle.
Spoon the onion chutney in a thin line parallel to the sausage meat, leaving a gap between the two.
Brush the edges of the pastry with egg, then roll up from each side so the pastry meets in the middle, holding the meat and chutney in two separate ‘tubes’. Press down well to seal the two tubes together and chill for at least 15 minutes or until the pastry is firm to the touch. Cut each pastry log into 10 pieces with a sharp knife, brush all over with a beaten egg, then scatter with sesame seeds.
Put on a baking sheet lined with nonstick baking paper, with the pastry join underneath, and put in the oven for 10-12 minutes or until golden brown. Serve hot, decorated with a sage leaf, if you like.