Add ¼ cornstarch to the marinated meat and mix/turn until well coated.
In a skillet/pan (8"), add oil and heat up to medium-high. Wait for at least 3 minutes.
Place each slice of beef in the skillet/pan (don't overcrowd the meat). If using a small pan you may need to cook/fry in two batches. (Watch the video for reference)
Cook each side for 2-3 minutes or until brown and crispy.
Remove cooked meat and transfer to a plate with paper towel; turn off the heat. Remove the oil and discard it (or you can save the oil if you prefer). Make sure to leave at least 1 tsp in the skillet/pan.
Heat the same skillet/pan up to medium-high.
Add garlic, onion and bell pepper. Stir and cook for 1-2 minutes or until fragrant. Remove the vegetables place them on a plate.
Add the sauce to the skillet/pan. Cook for 1-2 minutes or until it thickens alittle.
Return the cooked meat to the skillet/pan. Stir for several minutes or until most of the sauce has thickened.
Return the cooked vegetables and stir. Turn off the heat.