Heat a dry frying pan until hot and add the sausages. Cook for about 5-8 minutes until golden all over.
Meanwhile, heat the sunflower oil in a large casserole dish. Add the red onion, parsnips and swede. Season well. Cook over a medium heat for about 5-10 minutes until the vegetables are just starting to soften, then stir in the sage and cook for 1-2 minutes more.
Transfer the sausages to the casserole pan with the lentils, tomatoes, chicken stock and rosemary. Give everything a good stir, then cover and bring to a gentle simmer. Cook for 30 minutes over a low heat, stirring occasionally. Season to taste. Scatter over the parsley and serve with some crusty bread, if you like.
For top tips on protecting you and your family when preparing raw meat and poultry, visit Food Safety in the Home and view the full Food safety at home video.