Mix the water and cornstarch in a small saucepan over medium-high heat until thickened. Remove from heat, and mix in lemon juice, garlic, sumac, salt, and pepper. Refrigerate until ready to use.
In a large bowl, toss together the lettuce, cucumber, tomatoes, green onions, parsley, mint, bell pepper, purslane, and arugula. Toss with the dressing, and serve with pita.