Step 1In a heavy pot, heat the olive oil over medium heat until hot. Add the pancetta and cook, stirring, until the pancetta has rendered and begun to crisp, about 2 minutes. Stir in the onions, celery and carrot and cook until they are softened, 5 to 7 minutes. Add the lentils and broth and bring to a simmer. Cook, stirring occasionally, until the lentils are just cooked through but still have a little bit of bite, about 25 minutes. The lentils should absorb almost all of the stock. Season with 1/4 teaspoon salt and a few grinds of pepper, then stir in the butter and parsley. Slice the cooked sausage and serve atop the lentils.