In a bowl add ¼ cup cornstarch and season with a little salt and pepper. Stir to combine with a fork. Add beef pieces and toss to coat well.
Add all the ingredients for the sauce into a small bowl and whisk until well combined.
In a large pan over medium-high heat, add vegetable oil. Once the oil is hot, drop beef pieces and cook beef in one layer, do not overcrowd, cook until crispy and golden-brown. If needed cook in batches. Remove beef from the pan and set aside on a plate.
Add a bit more vegetable oil if needed to the pan and add the vegetables. Cook for 7-8 minutes, until tender and lightly browned.
Return beef to the pan and add the sauce. Stir and coat vegetables and chicken for 1 minute and turn the heat off.
Transfer to a plate or bowl and serve Hunan beef with a side of white rice.
Garnish with green onions and sesame seeds before serving.