In a medium bowl, toss beef strips with cornstarch, a pinch of salt, and pepper until evenly coated.
Heat 1 tablespoon of vegetable oil in a large wok or skillet over high heat.
Add half the beef in a single layer; stir-fry 1–2 minutes until just browned. Transfer to a plate and repeat with remaining beef.
Lower heat to medium-high; add remaining oil, garlic, bell pepper, broccoli, and carrots. Stir-fry 3–4 minutes until crisp-tender.
Return beef to the pan. Stir in soy sauce and brown sugar, tossing to coat all ingredients.
Cook 1–2 more minutes, allowing sauce to thicken slightly.
Tip: For an extra kick, add a teaspoon of chili garlic sauce or a dash of sesame oil.
Remove from heat and serve immediately over steamed rice or noodles.