Combine lentils and broth in a large saucepan over medium-high heat; bring to a boil. Reduce heat, and simmer 10 minutes or just until liquid is absorbed, stirring occasionally. Stir in barley and 2 3/4 cups water; bring to a simmer. Cook 18 minutes or until liquid is absorbed, stirring occasionally.
Transfer lentil mixture to a large bowl.
Add onion and next 5 ingredients (through garlic), stirring well to combine. Cool to room temperature. Cover and chill at least 1 hour before serving.
