Preparation

• Heat the oil in a 3-quart saucepan over medium heat. Add the curry paste and cook, stirring, until fragrant and a shade darker, about 2 minutes. Add 3/4 cup water, the tamarind, honey, fish sauce, and chili garlic sauce; bring to a simmer and cook until slightly reduced, about 5 minutes.
• In a small bowl, dilute the tapioca starch or cornstarch with 2 Tbs. water. Whisk into the sauce, and then cook, whisking, until thickened, about 2 minutes. Stir in the scallion and cilantro and sweeten to taste with more honey, if necessary. Remove from the heat and cool to room temperature (the sauce will thicken further as it cools). Skim any fat from the surface and discard. Garnish with the sesame seeds.

Make Ahead Tips

You can make the sauce up to 1 day ahead and refrigerate it in an airtight container. Serve at room temperature.