Heat a little oil in a heavy-based casserole dish and cook the sausages until browned all over. This should take about 10 minutes.
In the meantime finely slice the onion and finely chop the carrot, pepper, celery, garlic and the herb.
Transfer the sausages to a plate, add some more oil to the pan and sauté the chopped vegetables for 5-10 minutes. When they are soft stir in the lentils, add the tomatoes, wine and sugar. Let it sizzle for a minute then pour in the stock and the vinegar.
Put back the sausages, add the bay leaf and under a lid simmer for 30 minutes or so, until the lentil is tender (but still al dente).
Season to taste and serve with the chopped parsley.