Begin by slicing your choice of flank steak, sirloin, or ribeye into thin strips. In a medium bowl, combine the beef strips with soy sauce and cornstarch, ensuring each piece is well-coated. Allow this to marinate for about 15 minutes at room temperature.

Heat a tablespoon of oil in a large skillet or wok over medium-high heat. Once the oil shimmers, add in thinly sliced onions and stir-fry for about 3-4 minutes until they become translucent and slightly golden. Remove onions from the pan and set aside.

In the same skillet, ensure the oil is hot, then add the marinated beef strips in a single layer. Sear for 3-5 minutes, stirring occasionally, until browned and cooked through.

Lower the heat slightly and add minced garlic and grated ginger to the pan. Stir quickly for about 30 seconds, then pour in additional soy sauce and oyster sauce. Mix well to coat the beef.

Return the cooked onions to the skillet, along with your choice of colorful bell peppers. Toss all ingredients together and allow the peppers to slightly soften for about 2-3 minutes. Serve hot over rice.