Heat a medium skillet over medium-high heat and add the olive oil. When it begins to smoke lightly, add the onion and 1 tsp salt. Turn the heat up to high and cook, stirring from time to time, until the onions darken and char but do not burn, 5 to 8 minutes. Remove the skillet from the heat.
In a large salad bowl, whisk together the tahini, lemon juice, vinegar, and coriander. Add water as needed to make it a creamy dressing consistency (I needed almost ½ cup).
Stir in the chickpeas, onion, scallions, and more salt to taste. Serve immediately.