Thoroughly wash the romaine lettuce leaves in cold water. Dry completely using a salad spinner or clean kitchen towels.
Tear the lettuce into bite-sized pieces and place in a large serving bowl.
Slice the cucumber into thin rounds, halve the cherry tomatoes, and thinly slice the red onion.
Add the prepared cucumber, tomatoes, and red onion to the bowl with the lettuce.
Gently toss the vegetables together to distribute evenly.
Sprinkle the crumbled feta cheese over the salad.
In a small bowl, whisk together the olive oil and lemon juice. Season with salt and freshly ground black pepper to taste.
Just before serving, drizzle the dressing over the salad and toss gently to coat all ingredients evenly.
Serve immediately while the lettuce is crisp and fresh.