Slice any carrot that is larger than the circumference of your thumb lengthwise into halves or quarters. Line 2 rimmed baking sheets with parchment paper. Combine the oil, sriracha, honey and salt. Pour over the carrots and toss to coat completely. Arrange the carrots cut side down on baking sheets. Roast 40-50 minutes or until the carrots are browned on the cut side and slightly shrivelled looking. Switching the baking sheets from top to bottom rack while cooking to brown the carrots evenly. Let cool and arrange on a serving plate.
While the carrots are roasting, make the dressing. Slowly stir the water and lemon juice into the tahini, the consistency should be like ranch dressing; it will thicken with standing. You may need more or less water depending on the brand of tahini. Stir in the garlic and salt.
Drizzle on the carrots and sprinkle with the green onions and sesame seeds. Can be made a day in advance and refrigerated, covered. Apply the dressing and garnish just before serving.