Authentic Lebanese Fattoush

Name: Authentic Lebanese Fattoush
Ingredients: 0.5 cup water; 1 teaspoon cornstarch; 1/3 cup lemon juice; 2 cloves garlic, minced; 2 teaspoons sumac powder; salt and ground black pepper to taste; 1 head romaine lettuce, torn into bite-size pieces; 2 large tomatoes, diced; 1 green bell pepper, seeded and chopped; 1 medium cucumber, diced; 1 cup chopped purslane; 4 green onions, chopped; 0.5 (5 ounce) package arugula; 0.25 cup chopped fresh flat-leaf parsley; 0.25 cup chopped fresh mint; 4 pita rounds, toasted and torn into pieces