Batter: Whisk together the cornstarch and flour in a large bowl. Add the water and egg and whisk until the batter is smooth. Add the beef strips and stir until completely coated; set aside. Sauce: Whisk together the sauce ingredients in a medium bowl; set aside.
Fry Beef: Fill a large Dutch oven/fryer (this is the one I use) with 1 inch of vegetable oil. Heat the oil to 350-375 degrees F (177 C). Line a baking sheet with paper towels to receive the fried beef. Working in batches, use tongs to add the beef strips to the hot oil. Use a fork to stir and break them apart as they cook.
Fry the beef strips for 3 to 4 minutes or until golden (they will only be lighlty golden, not deeply golden due to the cornstarch). Remove the beef strips with a slotted spoon or spider strainer and let them drain in a single layer on the prepared paper towels. Simmer Sauce: Add the sauce to a large saucepan and simmer for 2 minutes, (it will not thicken, we are just concentrating the flavors). Combine: Reduce heat to medium-low. Add the fried beef strips and stir to coat in the sauce for about 2 minutes, until the beef absorbs the sauce. Garnish beef with the sesame seeds and green onions.