Melt the butter in a medium saucepan over low heat, add the scallions, and cook, covered, until tender, about 8 minutes. Add the cilantro, couscous, broth, 1/2 tsp. salt, and pepper to taste. Stir, bring to a boil over high heat, cover, and remove from the heat. Set aside for 5 minutes. Fluff the couscous with a fork. Taste for salt, add a large grinding of pepper, and season with some of the lemon or lime juice.