How to Make It
Place the flank steak in a sealable plastic bag and add the beer. Marinade in the fridge for 2 hours, occasionally shaking the bag.
While the steak is marinating prepare the sauce. Heat the sesame oil in a small saucepan over medium high heat. Add the ginger and garlic and cook for 1 minute. Add the soy sauce and water and bring to a boil. Add the brown sugar, molasses and Sriracha and continue to boil 5 minutes. Remove from the heat and set aside.
When the beef has finished marinating, bring a large pot of salted water to a boil. Add the chow mein noodles and cook according to package directions. Drain and rinse well under cold water. Using a clean kitchen towel remove much of the water from the noodles.
Line 2 plates with paper towels. Heat 2 tablespoons of the oil in an 8-inch cast iron pan or non-stick pan over medium heat. Add half of the noodles and spread them around evenly. Cook, without disturbing, for about 4 minutes, until light brown and crispy on the other side. Using a spatula lift a corner of the noodles. If the noodles have started to brown and crisp carefully flip the disc over. Remove the crispy noodles from the pan and place on the paper towel-lined plate. Add 2 more tablespoons of oil to the pan and repeat the process with the remaining noodles.
Remove the beef from the marinade and discard the marinade. Dry the beef with paper towels. Cut into thin strips cutting across the grain. Toss the strips with the cornstarch.
Line a baking tray or large plate with paper towels. Heat the remaining 2 cups of vegetable oil in a small saucepan with high sides over medium high heat. Attach a candy/deep fry thermometer to the side of the pot. Once the oil reaches 350° carefully place 3-4 pieces of the beef in saucepan. Cook for 1 1/2 minutes then remove with a slotted spoon and place on the paper towel-lined baking sheet. Repeat with the remaining beef, adjusting the heat to maintain the oil temperature at 350-375°.
Add the bok choy and 1/2 cup of water to a large frying pan over high heat. Steam for 3 minutes then remove from the heat.
Toss the beef with 1/2 cup of the sauce. Cut each crispy noodle circle in half and divide them amongst the plates. Top with bok choy and Mongolian beef. Serve the remaining sauce on the side.