Add all of the ingredients for the sauce to a bowl or measuring cup and whisk them together until combined (note: you can also blend everything in a small blender like a Magic Bullet if you have one). Set aside until ready to use.

Whisk the egg in a mixing bowl then toss the beef in the egg until all of the pieces are well-coated.

Add the flour and stir until the beef is coated in flour. Note that the beef will stick together at first, but it will come apart during the cooking process so no need to pull the pieces apart from each other.

Heat a large skillet over high heat and add the avocado oil. Once the skillet is very hot, add the breaded beef and spread it into a single layer. Allow the beef to brown for 3 to 5 minutes, stirring occasionally, until it has a golden crisp on it.

Transfer the onion and bell pepper to the skillet and continue stir frying for 3 to 4 minutes, or until the vegetables have softened.

Pour the sauce into the skillet and stir well until everything is well-coated.

Continue cooking, stirring occasionally, until the sauce is thick and sticky, about 5 to 8 minutes. Serve Beijing Beef with your choice of steamed rice, fried rice, or cauliflower rice.