In a small bowl, combine the soy sauce, brown sugar, rice vinegar, and garlic powder. Set aside.

In a saucepan, bring water to boil and cook broccoli for about 5 minutes or until it reaches desired doneness (remember it will cook a little more when you add it to the stir fry). Drain and set aside.

Meanwhile in a large skillet, heat half the olive oil until hot. Add the mushrooms and cook until they brown. Salt midway through, but not too much with the addition of the soy sauce later. Remove the mushrooms from the pan and set aside.

Add the remaining oil to the skillet, heat, and then add in the beef. Cook the beef until it browns nicely. Work in two batches if necessary as to not crowd the pan.

Return all the beef to the skillet with the broccoli and mushrooms. Pour sauce over the top and stir to combine. Heat through and serve over cooked rice.