Season the thinly sliced flank steak with salt and pepper (about 1 teaspoon of each) and set aside.
In a small bowl, combine all of the sauce ingredients and whisk well until combined. Set aside.
In a large skillet or wok, heat 1 tbsp of the olive oil over medium-high heat. Add the bell peppers, sliced onion, and green beans. Cook the veggies until tender and slightly browned, about 5-7 minutes. Remove the veggies to a bowl.
Add 1/2 tbsp of olive oil to the skillet and add half of the flank steak. Sear for 1-2 minutes and then flip and sear the other side. Do not overcook. You want to aim for a nice sear on the meat.
Remove the seared steak to a bowl. Add another 1/2 tbsp of olive oil to the skillet and add the remaining steak and cook the same as the first half.
When all of the steak has been cooked, add the first half back to the skillet, along with the veggies.
Give the sauce ingredients one last whisk and then pour into the skillet as well. Lower the heat to medium and toss the steak and veggies in the sauce until the sauce has thickened, about 2-3 minutes.
Optional: top with toasted sesame seeds or sliced green onion. Serve over rice, if desired. Enjoy!