Lentil Salad with Arugula and Pickled Sweet Peppers

French lentils du Puy are meaty and firm even when fully cooked, so they’re great in salads. In this recipe, we balance their earthy flavor with the sharpness of vinegar and Dijon mustard, the sweetness of Peppadew peppers (sweet cherry peppers work nicely, too) and peppery arugula. Serve at room temperature as a side to roasted or grilled meats or seafood or as a light vegetarian main.