Make the sauce: In a small bowl, whisk together 1/2 tsp pepper, 1/2 tbsp lemon juice, 2 tsp mirin, 2 tbsp soy sauce, 1/2 tbsp oyster sauce, 1 tbsp rice vinegar, 1 1/2 tsp hoisin sauce, 1 tbsp cornstarch, 1 fl oz water, 1 1/2 tsp sesame oil, and 1 tsp brown sugar. Set aside.
Marinate the beef: In a small bowl, add 14 oz beef flank, 1 tbsp cornstarch, and 1/2 tbsp olive oil. Toss well to coat the beef, then allow it to sit for 10 minutes.
Stir-fry the vegetables: Heat 1 tbsp olive oil in a skillet over medium heat. Add 2 oz carrots, 4 oz broccoli, 2 oz snow peas, and 2 oz red bell pepper. Stir-fry for 3 minutes. Transfer to a plate.
Stir-fry the beef: Heat the remaining 1/2 tbsp olive oil in the skillet, add 2 tbsp garlic and 1 tsp ginger. Stir-fry for 30 seconds until fragrant. Add beef chunks and stir occasionally for 1 minute before adding vegetables. Stir to combine and cook for another 3 minutes.
Toss the sauce: Reduce the heat to low and pour in the sauce. Toss to combine everything and cook for an additional 2 minutes. Remove from heat.
Garnish: Top with 0.5 oz scallion.
Serve: Divide the stir-fried dish into 4 serving plates. Serve immediately over steamed white rice.