Prepare the Vegetables: First, wash and dry your produce. Dice the English cucumber and the seeded Roma tomato into small, uniform ¼-inch (6 mm) cubes. Mince the serrano chile very finely.
Chop the Fresh Herbs: Gather the fresh mint leaves, roll them together tightly, and thinly slice them before giving them a rough chop. Set aside.
Start the Dressing Base: In the bottom of your final shallow serving bowl, combine the tahini paste, grated garlic, and fresh lemon juice. Whisk them together; the mixture will thicken considerably.
Emulsify the Tahini Sauce: Slowly drizzle the ice-cold water into the tahini mixture while whisking continuously. Continue until the sauce becomes smooth, pale, and has the consistency of creamy yogurt.
Season the Dressing: Stir the fine sea salt into the emulsified dressing. Taste and adjust with another splash of lemon juice if you prefer more brightness.
Combine and Garnish: Add the diced cucumber, tomato, and minced serrano to the bowl with the finished dressing. Add most of the chopped mint, reserving a pinch for garnish. Gently fold everything together until the vegetables are evenly coated. Sprinkle the reserved mint and a dusting of sumac over the top before serving.