This egg sandwich does more than use up leftover hard-boiled eggs. It’s also a great way to put leftover fresh herbs to work—you know, the ones in the crisper drawer, languishing in produce bags. For our recipe, we used one tablespoon of flat-leaf parsley; basil, dill, tarragon, and chives would work well, too. Simply chop up the greens, mix them into a lightened-up mayonnaise-mustard mix, and schmear it over one slice of bread. Layer the other side with sliced eggs and watercress, which lends crunch and even more flavor. All that’s left is a shake of salt and a few cracks of pepper, and you’re ready to eat (or tuck it away for a filling lunch).