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Ask the butcher to cut beef into paper-thin slices, then you can place the beef in the freezer for 30 minutes to firm it until tight, but not frozen. Cut slices all together diagonally into about 2-inch strips. Place it in the medium-sized bowl, then add half onion (rings), garlic, and season it with salt and ground black pepper to taste (about 2 tsp. Salt and 3 tsp. Ground black pepper). Pour oil and Rice vinegar, and mix it all up to coat the thinly sliced beef. Cover with a plastic wrap and place it in the fridge to marinate for about 30 minutes.
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While beef is marinating, slice green pepper, and remaining onion. Make sure to separate rings.
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Add oil in the nonstick pan and turn the heat on medium high. Add marinated beef first, saute for about 5 minutes, then add green pepper and onion. Season with more salt and ground black pepper if needed.
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Saute for 15 minutes or until green pepper is completely done, and onion starts to caramelize. I like to cover it with a fitted lid, so the peppers could steam up, and I lift the lid every few minutes to stir. You can add more oil if necessary. (But 3 tbsp. Should be enough.)
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Wrap it up with baby spinach, and tomato. (Or any other topping such as lettuce, pickles, etc.)