Heat the oil (if using) in a large pan and fry the sausages until golden. Remove and set to one side. Then add in the onion and carrots to the pan and cook for 5-10 minutes, over a medium heat, until soft, adding in the garlic (if using) and chilli for the last minute.
Stir in the chopped plum tomatoes and tomato purée, with 250ml stock (if using) and bring to a boil. Simmer for 10 minutes and then stir through the sausages. Cook for a further 15-20 minutes.
Meanwhile, place the couscous in a bowl and add 400ml boiling water. Cover with clingfilm and then leave for 5 minutes. Fluff up with a fork and stir through the dried mixed herbs (if using) before serving.
Cook's tip: for vegetarians, try swapping the pork sausages for 2 x 300g packs of vegetarian sausages.