Puree the onions and garlic in a food processor until smooth, adding a splash of water if needed. Pour the puree over the beef and let sit it at room temperature for 45 to 60 minutes. (This step is to tenderize the meat. If you are worried about leaving meat out, place it in the refrigerator for 2 hours.)
Whisk the soy sauce, brown sugar, vinegar, corn syrup, sugar, sesame oil, ginger, cola, apple soda and 1/2 cup room temperature water until well incorporated. Pour the mixture over the beef and marinate in the refrigerator for 24 hours.
Set a skillet over high heat and get the oil scorching hot. Drop in the beef and listen for a sizzle when it hits the pan. (We are looking for a sear on the bottom, so do not touch the meat for first 2 minutes; let the pan do the work.) Once the edges of the meat are brown, mix it up while scraping the browned bits off the bottom of the pan. Cook until the meat juices have reduced by half and been absorbed into the beef, about 10 minutes.
Set up a steamer and steam the frozen gua bao buns until they have a soft, pillow-like texture, 3 to 5 minutes. Carefully open the steamed buns and drizzle some spicy aioli inside. Add a layer of baby greens and then, with a spoon or tongs, place the beef on top of the greens. Garnish the beef with tempura onions.
Mix the mayonnaise, hot sauce, sour cream, corn syrup, sambal, honey, sesame oil and garlic in a bowl. Whisk until smooth. (Add half the amount of hot sauce and double the sour cream for milder heat.)
Heat the oil in a pot to 350 degrees F for frying.
Season the buttermilk with salt and pepper and set aside. Whisk the tempura flour, sprite, and 1/4 cup ice-cold water to get a pancake batter consistency. Dip the onions in the buttermilk, then coat them with the batter. Fry the onions until amber brown, then drain on a paper towel-lined plate. Season lightly with salt and pepper.
It is important that the liquids are as cold as you can get them, as the reaction of ice-cold batter in the hot oil makes the onions extra crispy.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.