Heat oven to 200C/180C fan/gas 6. Sit the chipolatas in a roasting tin and cook for 15 mins.
Meanwhile, pop open the crossiant packs and unroll. Mix the onion marmalade, tomato puree and mustard, and spread some over each dough triangle. When the chipolatas are done, set aside for 5 mins to cool while you line 2 baking sheets with baking parchment. Lay a sausage along the wide end of each triangle croissant and roll up, encasing the sausages. Arrange on the sheets, brush with beaten egg and scatter with sesame seeds. Bake for 10-15 mins, switching shelves halfway, so both sheets cook evenly, until golden.