Chop broccoli into small florets. And slice onion into thin slices. Set aside.

Chop beef into 1-inch cubes. In a large bowl, mix cornstarch with 3 tablespoons of water. Add beef to the bowl and mix until pieces are evenly coated.

Add 2 tablespoons olive oil to a large pan over medium high heat. Add beef and cook, stirring occasionally, until almost cooked through, but not fully cooked yet. Remove beef from the pan and set aside.

Add remaining 1 tablespoon olive oil to the pan. Add sliced onion and cook, stirring occasionally, for about 5 minutes, until soft. Add broccoli to the pan and cook for 5 more minutes, until soft.

Add teriyaki sauce to the pan and return beef to the pan. Mix well to combine. Let simmer for about 5-10 minutes, until everything is evenly coated and sauce has thickened. Serve with brown rice.