In a large salad bowl, combine the diced tomatoes, cucumber, green bell pepper, red onion, parsley, mint, and radishes.
Add the toasted pita bread pieces to the salad bowl. You can toast the pita bread by placing it in the oven or toasting it in a dry skillet until crisp. Break the toasted pita into small pieces and add it to the salad.
In a small bowl, whisk together the lemon juice, olive oil, sumac (if using), salt, and pepper to make the dressing.
Pour the dressing over the salad and toss gently to combine, ensuring all the ingredients are coated in the dressing.
Taste and adjust the seasonings if needed. You can add more lemon juice, olive oil, salt, or pepper according to your preference.
Let the salad sit for about 10-15 minutes before serving to allow the flavors to meld together.
Serve the Fattoush salad as a refreshing side dish or as a light and healthy main course.