BRING water to boil in medium saucepan. Stir in couscous; cover. Remove from heat; let stand 5 min. Fluff couscous with fork; cool, uncovered, 10 min.
MIX couscous, lemon juice and oil in large bowl. Add tomatoes and onions; cover. Refrigerate at least 4 hours or up to 24 hours.
STIR in cheese just before serving. Serve on lettuce-covered plate or as a filling for pita bread.