Rinse the pearl barley thoroughly under cold water.
In a medium saucepan, combine barley and water (or broth). Bring to a boil, then reduce heat to a simmer, cover, and cook for 25-30 minutes until tender but still chewy.
Drain any excess liquid and spread the barley out to cool completely.
While the barley cools, prepare your vegetables by dicing the cucumber, halving the cherry tomatoes, and finely chopping the red onion.
In a large bowl, combine the cooled barley, cucumber, tomatoes, red onion, and parsley.
In a small bowl, whisk together the olive oil, lemon juice, salt, and pepper until well combined.
Pour the dressing over the barley mixture and toss gently to coat everything evenly.
Sprinkle the crumbled feta cheese on top of the salad.
Let the salad rest for at least 15 minutes before serving to allow the flavors to meld.
Serve chilled or at room temperature.
