Heat the oil over medium heat in a large heavy-bottomed pot or dutch oven. When hot, add the onion, carrot and celery. Saute for five minutes, stirring frequently, then add the garlic and continue sauteing for another two minutes.
Add the fresh rosemary, dried thyme, bay leaf, smoked paprika, salt, pepper and lentils. Mix well then pour in the stock. Bring to a boil, then reduce heat and simmer for about 25 minutes until lentils have absorbed most of the liquid. Stir in the Dijon mustard and red wine vinegar and continue simmering until all the liquid has been absorbed. Adjust seasonings with salt and pepper.
Meanwhile, preheat the oven to 425 degrees. Place sausages on a foil-lined baking sheet and lightly mist with cooking spray. If you don't have cooking spray, brush lightly with olive oil. Place in the oven and roast for 15-20 minutes, flipping midway through. The sausages should appear to be crispy and golden.
When the sausages are finished roasting, place in the pot with the lentils and serve together.