Cut the flank steak into ¼ inch (0.6 cm) strips, cutting against the grain. If it's too difficult to slice thin, place it in the freezer, uncovered for 20-30 minutes to firm up.

Add the sliced beef to a large bowl with the vegetable oil, oyster sauce, cornstarch, and baking soda. Toss to combine and set aside at room temperature for about 30 minutes.

For the sauce, add all the ingredients to a 2-cup liquid measuring cup or medium bowl. Whisk to combine and set aside.

In a large non-stick pan set over medium-high heat, add ¼ cup (60 ml) of water along with the broccoli florets. Cover with a fitted lid and steam for 4-5 minutes until tender. Set said in a bowl and wipe out any excess water with a paper towel.

To the same pan set over medium-high heat, add ½ tbsp of vegetable oil to the pan and spread half the beef in an even layer. Sear the meat on both sides for 2-3 minutes until browned. Set aside on a plate and do the same with the remaining oil and beef.

Reduce the heat to medium and add the sauce (give it a quick whisk if the ingredients have settled). Stir constantly for 1-2 minutes until thickened.

Turn off the heat and add back the beef and broccoli. Toss to coat everything in the sauce and serve warm over rice. Sprinkle on some sesame seeds if desired.