This recipe came from a cooking class my mom took many years ago (possibly before I was born). For several years, I've had a scanned copy of the photocopied recipe with her notes from the cooking class. We finally replicated the Mongolian Beef recipe. While we intend to play around with the sauce in future iterations, the "velveting" method of cooking the beef was a big success and will definitely be repeated.

Ingredients

Velveting:
1.5 lbs flank steak
3 egg whites
2 tsp white wine or sherry
Black pepper
1/3 cup cornstarch
Vegetable oil

Sauce:
3/4 cup soy sauce
4 1/2 Tbsp sugar
3 Tbsp sesame oil
3 Tbsp white wine or sherry
3 tsp rice vinegar
2 tsp Hoisin sauce
9 cloves garlic, minced
1 tsp chili oil

Garnish:
6 green onions

Preparation:
Put beef in freezer for approximately 20 minutes before slicing to enable thinner slices.
(Meanwhile, you can mix the sauce ingredients together.)
Slice beef into 1/8" thick slices (basically as thin as you can)

Mix together egg whites, sherry, black pepper, and corn starch. Mixture should be goopy.
Add sliced beef to mix and toss to coat.

Heat oil to 250 degrees.
Working in batches (likely at least 4), deep fry beef for approximately 10 seconds (or until gray with some browning).
Drain beef on a paper towel.

Pour mixed sauce ingredients into another pan.
Cook 2-5 minutes or until slightly reduced.
Add beef; toss to coat.

Serve garnished with sliced green onions.

Do ahead:
Beef can be fried with the cornstarch breading several hours before.
Re-crisp in an air fryer or toaster oven before adding to sauce.