Heat alittle of the oil in a large frying pan over medium heat. Fry the sausages for 5–6 minutes, or until browned all over. Add the pancetta halfway through cooking the sausage. Remove from the heat and set aside.
Heat the remaining oil in a large heavy-based saucepan and add the onion, leek, garlic, celery and carrot. Stir for 2 minutes to coat the vegetables in the oil. Reduce the heat, cover and simmer, stirring occasionally, for 10 minutes. Do not allow the vegetables to brown.
Add the thyme and stock. Slowly bring to the boil, then reduce the heat and simmer, covered, for 20 minutes. Remove the thyme. Stir in the beans, then remove the saucepan from the heat.
Roughly purée the soup in a blender and return to the pan. Dice the sausages and add them to the soup along with the pancetta and tomatoes. Gently reheat, stir in the parsley and season with salt and pepper.