Heat oven to 350° F.
Place the pita rounds on a baking sheet and toast in oven until crisp and light brown, about 10 minutes. Break into bite-size pieces and set aside.
Toss together the cucumber, tomatoes, scallions, fennel, celery, mint, and parsley. Cover and chill.
Meanwhile, in a small bowl, whisk together the oil, lemon juice, garlic, salt, and pepper.
Combine the vegetables and dressing and fold in the pita pieces. Serve with the hummus (if desired).