To make the dressing, combine the lemon juice, garlic, cumin, sumac (if using), salt, red pepper flakes and black pepper in a blender or a food processor. Process until smooth. With the motor running, slowly add the olive oil in a thin stream until emulsified. Set the dressing aside.
Preheat the oven to 400 F. Spread the pita bread pieces in a single layer on a baking sheet and bake until crisp and lightly golden, about 8 minutes. Set aside to cool. In a large bowl, combine the lettuce, tomatoes, cucumbers, bell pepper, green onions, mint and parsley and toss. Add the dressing and toss lightly to coat evenly. Divide the salad among individual plates. Top with the pita croutons, dividing evenly. Serve immediately.