Blanch beans (optional): Bring a pot of water to boil. Blanch green beans for 2 minutes, then drain and pat dry. (This step keeps beans crisp and bright.)
Sauté garlic & chili: Heat oil in a large skillet or wok over medium-high heat. Add garlic and dried chilies, stir-fry for 30 seconds until fragrant.
Add beans: Toss in green beans and stir-fry for 5–7 minutes until tender-crisp.
Season: Stir in soy sauce, vinegar, sugar (if using), salt, and pepper. Toss well to coat.
Serve: Garnish with sesame seeds or scallions. Serve hot.