2 tablespoons canola oil
2 medium cloves garlic, minced (about 2 1/2 teaspoons)
1/4 cup soy sauce
2 tablespoons light brown sugar
1 1/2 tablespoons ketchup
1 tablespoon distilled white vinegar
1 tablespoon toasted sesame oil
2 teaspoons Thai chili sauce, such as Sriracha
1/2 cup plus 1 tablespoon water
2 teaspoons cornstarch
3 scallions (both white and light-green parts), trimmed and cut crosswise into thin slices (1/4 cup)
2 teaspoons sesame seeds, toasted
Combine the oil and garlic in a medium saucepan over medium heat; cook for 1 to 2 minutes, until the garlic sizzles steadily and becomes fragrant.
Add the soy sauce, brown sugar, ketchup, vinegar, toasted sesame oil, chili sauce and 1/2 cup water. Increase the heat to medium-high to bring just to a boil.
Meanwhile, combine the cornstarch with the remaining 1 tablespoon water in a small bowl; stir it into the bubbling sauce. Return to a boil so the sauce thickens, then add the scallions and sesame seeds. Cover and refrigerate for up to 5 days.
NOTE: Toast the sesame seeds in a dry skillet over medium heat for 3 to 5 minutes, stirring or shaking them frequently, just until fragrant and lightly browned.