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1
Trim the excess fat and cut the flank steak in small, bite-size pieces. Combine the ingredients for the marinade and let the beef marinate for at least 20 minutes.
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2
Meanwhile, cut the onions in thin slices. The bell peppers should be seeded and cut in 1-inch slices. Cut the tougher ends of the asparagus and slice them on the diagonal into 2-inch long pieces.
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3
Bring a medium pan of water to a boil, reduce to a simmer and throw the asparagus in for about 2 or 3 minutes. Drain them, run them under cold water and dry them.
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4
In a wok or saute pan over a high flame, heat 1 tablespoon of the oil. Add the marinated beef strips, making sure it makes a nice sizzling sound. Once the meat is nicely browned on the outside, but still tender in the middle, about 2-3 minutes depending on the size, take it out and set it aside on a plate. You may need to sear the beef in two batches if your wok is not big enough.
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5
Add 1 more tablespoon of the oil, and briefly stir-fry the asparagus for 3-4 minutes at medium-high eat. Set the asparagus aside and keep warm.
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6
Add another tablespoon of oil, if necessary, and add the onions. Toss and turn until they have a nice golden brown color, 7 to 10 minutes. Add the peppers and stir until softened, about 4-5 minutes. Add sugar and vinegar, and cook for a few more minutes until the onions begin to caramelize.
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7
Add the stock and bring to a simmer cooking for another 2 minutes to reduce the liquid. Mix the cornstarch with the 3 tablespoons of water and add slowly add it to the mixture until the sauce is a thick as you desire.
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8
Return the beef and the asparagus to the pan and stir for another minute or so to amalgamate the flavors. Serve immediately with some white or brown rice.
