In a small bowl, combine the hoisin sauce, water, soy sauce, and sugar.
Place a wok or skillet over medium-high heat. Add olive oil and heat for 30 seconds. Add scallions, garlic, and pepper flakes and cook until fragrant, about 15 seconds.
Add the hoisin mixture and bring to a boil. Cook, stirring occasionally, until the sauce thickens slightly, about 1 1/2 minutes. Pour into a bowl.