Marinate beef: Combine flank steak with soy sauce, garlic, and cornstarch. Let sit for 15 minutes.

Heat oil: In a large pan, heat canola oil until hot. Test with a small piece of beef.

Fry beef: Cook the beef slices until golden brown. Remove and drain on paper towels.

Prepare sauce: In the same pan, discard excess oil, leaving about 1 tablespoon. Cook garlic and ginger for 30 seconds.

Cook zucchini: Add zucchini to the pan, stirring until it softens.

Add sauce ingredients: Stir in soy sauce, dark soy sauce, brown sugar, honey, Shaoxing wine, and rice wine vinegar. Let simmer until sauce thickens.

Combine: Return beef to the pan, toss to coat in sauce, and add chili flakes for heat.

Garnish: Top with sesame seeds and chopped green onions. Serve hot.