Place pearl onions in a microwave-safe bowl; add water. Cover and microwave on high for 1-2 minutes or until crisp-tender; drain and peel.
In a serving bowl, combine the red onion, cucumber, red pepper, tomatoes, green onions and pearl onions. In a small bowl, whisk the oil, vinegar, garlic, oregano, salt and pepper. Pour over vegetables and toss to coat. Just before serving, sprinkle with feta cheese.