In a large saucepan bring water and salt to boiling. Remove from heat. Stir in couscous; cover and let stand for 5 minutes. Fluff couscous with a fork.
Meanwhile, combine olive oil and lemon juice. Add oil mixture to couscous, stirring to coat. Let stand until completely cool.
Stir in spinach, green onions, and snipped dill. Cover and refrigerate for 2 to 12 hours or until thoroughly chilled. Sprinkle with feta cheese before serving. Serve with lemon wedges and top with fresh dill sprigs, if desired. Makes 8 to 10 servings.