Prep the steak: First we are going to want to examine the flank steak to find the direction of the grain. Once you found it make one slice through the middle of the flank steak that is parallel with the grain. This should leave us with two pieces. Next, you use a sharp knife to cut the flank steak into thin strips perpendicular to the grain. Transfer these thin strips of steak to a bowl and cover them with the marinade ingredients. Stir it all together and let the beef marinate for at least 30 minutes.
Add all of the ingredients for the sauce to a small bowl or cup and stir it all together. Set this aside until step 5.
Brown the beef: After the beef has marinated for 30 minutes get out a wok or large walled skillet and add about 3 tablespoons of vegetable oil to it. Place this over high heat, toss in the beef and stir fry it until the beef is browned. You might need to do this in batches. Once the beef is browned transer it to a plate and set it aside until later.
Stir fry the veggies: Add another tablespoon of oil to the wok and toss in the minced garlic. Stir fry this for 30 seconds making sure that the garlic doesn’t get burnt. Then toss in the bell peppers, Fresno peppers, and onion, stir-frying everything for 3 to 4 minutes or until the peppers begin to brown up.
Finish the dish: Toss the beef back into the wok and continue to stir fry everything for another minute. Then pour in the sauce we made in step two and stir until the sauce has thickened up and coated everything. This last part should take a couple of minutes.
Serve the bell peppers & beef immediately and enjoy!