Whisk together 1/4 cup lemon juice, 1/4 cup olive oil, 1/4 cup tahini, 1 teaspoon kosher salt, minced garlic, and pinch cayenne, (if using) in a large bowl. Note that it will be thin at first, then quickly become very thick with continued whisking. Also note that it will taste very salty - it will be be just the right amount of seasoning for the unseasoned vegetables.
Add the diced tomatoes, cucumber, and 1/2 cup coarsely chopped parsley to the bowl with the dressing and toss until well mixed. Garnish with sumac and/or additional parsley if desired. Best if eaten immediately, but leftovers can be stored in an airtight container in the fridge. Note that the salad will become more watery over time, but it will still taste great.