Heat the oil in a very large skillet or Dutch oven over medium heat. Be sure it has a lid. Add the bacon and sausages. Cook, stirring occasionally, until the bacon is crisped and the sausages are browned, about 10 minutes. Remove to a paper towel-lined plate.
Into the hot fat, add the onion, carrots, celery and garlic. Hit with generous pinches of salt and pepper. Stir, scraping the bottom, for a few minutes, until the onions are softened.
Add the lentils, then pour in two cups of chicken stock. Add the tomatoes and their juices, breaking them up with your hands or kitchen scissors. Stir in the mustard, apple cider vinegar and rosemary, and season with salt and pepper. Chop the cooked bacon and slide it into the skillet along with the sausages. Bring to a boil, cover, then turn the heat down to low and simmer for 45 to 50 minutes, stirring occasionally. You may need to add more chicken stock as the lentils simmer. You want them to be very tender.
Stir in the parsley and season to taste with salt and pepper. Serve with crusty bread and a salad.