Slice flank steak against the grain into thin, 2-inch pieces.
Heat the oil, garlic and ginger in a large skillet on medium for 1 minute. Add in steak and turn heat to medium-high. Cook for 5 minutes, then add veggies (saving green onion for last), cover and let cook for another 5-7 minutes.
Add in sauce ingredients, excluding arrowroot starch. Once the liquid in the pan gets hot, add a few tbsp into a small bowl along with the arrowroot, and stir until it's fully dissolved. Pour into the skillet, stir together, reduce heat, and let cook for another 4-5 minutes.
Add chopped green onion and sesame seeds over the top. Serve with jasmine rice.