How to Make It
In a large, heavy Dutch oven, heat 2 tablespoons of the olive oil. Add the garlic sausage and cook over moderately high heat, stirring, until browned, 5 minutes. Using a slotted spoon, transfer the sausage to a plate.
Add the leeks, carrot and garlic to the Dutch oven and cook over moderate heat until softened, about 7 minutes. Add the lentils, herb bundle and 5 cups of water and bring to a boil. Season with salt and simmer over low heat, until the lentils are tender, about 35 minutes. Discard the herb bundle.
Meanwhile, preheat the oven to 350°. Spread the hazelnuts in a pie plate and toast for about 12 minutes, until golden. Let cool, then transfer the nuts to a kitchen towel and rub off the skins. Coarsely chop the nuts.
In a small bowl, combine the mustard with the remaining 2 tablespoons of olive oil. Add the escarole to the stew and cook until wilted, 6 minutes. Stir in the sausage and cook until hot. Transfer the stew to bowls and drizzle with the mustard oil. Garnish with the nuts and parsley and serve.