How to Make It
In a large saucepan, melt the butter. Add the onion, celery and carrots and cook over moderately high heat, stirring, until softened, about 5 minutes. Add the lentils, stock and water and bring to a boil. Simmer over moderate heat until the lentils are tender, about 25 minutes. Season with salt and pepper. Add the ham and cook just until heated through, about 2 minutes. Drain well and return the lentils and ham to the saucepan.
Meanwhile, in a small bowl, whisk the mustard with the vinegar and garlic. Whisk in the grapeseed and olive oils and season with salt and pepper. Stir the dressing into the lentils and fold in the parsley and chives. Serve warm or at room temperature.