Season beef with salt and pepper. Mix using your clean hands.
Add 4 tablespoons cornstarch. Continue to mix until all ingredients are well blended. Let it sit for 12 to 15 minutes so that the meat can absorb the flavors from the seasonings.
Heat oil on a wok or pan. Fry one side of the beef for a maximum of 1 minute. Turn it over to fry the other side using the same maximum time. Remove the fried beef from the wok and arrange in a plate lined with paper towels. Set aside.
Start to make the Mongolian beef sweet sauce by pouring 3 tablespoons of oil used to fry beef on a wok. Quickly saute garlic and ginger.
Pour soy sauce. Let boil.
Add brown sugar. Stir until sugar dissolves completely.
Pour water. Stir and let boil. Continue to cook in medium heat for 3 minutes.
Combine 1 tablespoon of cornstarch with 1/4 cup water. Stir until well blended. Pour half of the mixture into the sauce. Stir until sauce thickens. Add the remaining cornstarch mixture if the sauce needs to be thickened more. Pour sauce into a bowl. Set aside.
Heat 2 tablespoons of the oil used to fry beef on a clean wok. Stir fry the fried beef for 30 seconds. Add half of the green onions. continue to stir fry for a minute.
Pour Mongolian beef sauce into the wok. Stir-fry until the beef slices are completely coated with sauce. Add remaining green onions. Continue to cook for 1 minute.
Transfer to a serving plate. Serve!
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