In a large dutch oven, heat 2 tablespoons of the olive oil. Add the sausage pieces, and cook over medium high heat until browned, about 5 minutes. Remove sausage to a plate.
Add the leeks, carrot, and garlic to the oil in the pan and cook over medium heat until softened, about 7 minutes. Add the lentils, herbs, bay leaf, and 5 cups water; bring to a boil. Season with salt and pepper, and simmer over low heat until the lentils are tender, about 35 minutes. Remove the herb sprigs and bay leaf.
In a small bowl, combine 2 tablespoons of olive oil with the mustard. Add the sausage back into the pot, and stir until sausage is warmed through. Transfer the stew to bowls, and drizzle with the mustard oil and parsley to serve.