In a medium saucepan, combine the broth, lemon zest, lemon juice, scallions, and salt. Bring to a boil over medium-high heat and cook for 2 minuets to soften the scallions and flavor the broth. Meanwhile, cut the scallops horizontally into thirds. Add the scallops and dill to the soup and cook for 30 seconds. Remove the pan from the heat so scallops don't overcook. Ladle the soup into 4 shallow bowls and garnish with dill springs, lemon slices and lemon zest, if using.