Melt butter in a large dutch oven over medium-high heat. When melted, brown the sausages on all sides, then remove from the pan and reserve. Add onion, garlic, carrot and celery, then cook until vegetables begin to soften, about 5 minutes. Add lentils and stir until everything is evenly incorporated, then nestle sausages back in the pan with the lentils and vegetables. Pour 2 cups of stock over sausages and bring to a simmer.

Cover and turn heat to medium-low. Let the mixture cook for 20 minutes. If lentils are still tough, add remaining stock and continue to cook until lentils are tender.

When lentils are tender, one from heat and serve sausages topped with a heap of lentils and a big dollop of mustard.