For the beef, if using flank steak: Cut the steak into very thin slices ACROSS the grain. If using other steak: Cut the steak into very small cubes, about 1/2-inch. If using ground beef: leave as-is.

In a large bowl, combine the soy sauce, sesame oil, ginger, garlic and brown sugar. Add the beef and toss to coat. Let marinate while you prep the vegetables. Steps 1 and 2 can be done ahead of time up to 1 day before.

Heat a large saute pan or wok on medium heat. When hot, swirl in the oil. Add the onions and saute for 1 minute. Turn the heat to high and give it just a few seconds to heat up. Add in the beef, spreading the beef all over the surface of the pan, tossing with the onions. Saute for 2-3 minutes until just barely pink.

Add in the carrots, zucchini and toss. Cook for 1 minute. Toss in the spinach and cook for an additional minute. Sprinkle with sesame seeds.