In a large bowl, toss beef and cornstarch. Place in the fridge for 15 minutes to allow coating to stick better.
In a small sauce pot, heat 2 tablespoons of vegetable oil over medium heat. Saute garlic and ginger until fragrant, about 2 minutes. Add water, soy sauce and sugar and stir to combine. Bring to a boil for about 3 minutes. Remove from heat.
Heat wok over medium high heat, add ½ cup of vegetable oil. Add beef in batches to sear the outside but barely cooked in the middle. Remove and let drain on a paper towel. Pour out any excess oil and scrape to clean the wok.
Return the wok to the burner over medium heat and pour in the sauce. It should come to a boil immediately. Add the beef and stir to coat. Continue cooking for 2 minutes, stirring constantly until sauce is thickened. Add the green onions. Garnish with roasted sesame seeds. Serve
Recipe by The Gluten Free at https://www.theglutenfree.com/gluten-free-mongolian-beef/