Marinate the beef by slicing it thinly against the grain, mixing with soy sauce, cornstarch, and sesame oil. Let it marinate for 15 minutes.

While the beef marinates, slice onions into thin wedges, chop red bell pepper into pieces, and mince garlic.

Heat vegetable oil in a skillet over medium-high heat. Cook marinated beef in batches for 2-3 minutes per side until browned. Remove and set aside.

In the same skillet, sauté onions for about 5 minutes until caramelized. Add bell pepper and garlic and cook for another 3 minutes.

Return the beef to the pan. Combine more soy sauce and cornstarch with water, pour over the stir-fry, and stir for 2 minutes until the sauce thickens.