For the lentils:
In a large pot heat up 3 tablespoons of olive oil. Add the chopped onion, carrot and celery, as well as pancetta if using. Fry for about 10 minutes until softened.
Prick the sausages with a fork or small sharp knife, add them to the pot and fry on a medium heat on both sides until golden in colour.
Now pour in the wine and wait until it has almost evaporated. Next pour in the passata, season with some salt and pepper. Add the marjoram as well as little sugar. Give everything a stir, lower down the heat and cook covered for about 30-40 minutes, or until the sausages are cooked through. Taste and season again.
In the meantime place the lentils in a small pot and cover with cold water (about 4cm above the level of the lentils).
Add the gently crushed garlic cloves and the celery stick cut in half. Don’t cut the celery or the garlic into smaller pieces as they will have to be discarded after the cooking.
Cover the pot with a lid and cook gently until almost done. It should take about 20-30 minutes. The lentils should be al dente, soft but still retain some crunch.
Drain the lentils but leave some of the cooking liquid for later if needed. Discard the garlic and celery stick.
Add the lentils into the pan with the sausages. Give it a good stir and cook together until the lentils turn soft. If the sauce seems dry, add a little bit of the reserved liquid after cooking the lentils.
Taste and season again. Add more marjoram if you prefer.
Serve the dish warm, garnished with some fresh parsley and lightly drizzled with a good olive oil.