Preheat the oven to 180c/160 fan assisted ovens/gas mark 4. Line a baking sheet with baking paper.
Heat the oil in a non-stick frying pan and add the onions. Sprinkle over the sugar and cook over a medium heat, stirring occasionally for 6-8 minutes or until golden and caramelised.
Open the croissant can and unroll the dough. Separate the dough into 6 triangles according to packet instructions.
Prick each chipolata once or twice with a fork or the tip of a sharp knife. Place the chipolatas at the shortest edge of each dough triangle. Divide the caramelise onions between the triangle, spreading over the sausages.
From the shortest edge, roll up the dough, over the filling to make croissants. Place on the baking sheet and brush with beaten egg.
Bake for 15-20 minutes until golden and sausages are cooked through (check cooking times for sausages on pack). Serve hot.