Mongolian Beef

Slice the flank steak into thin strips against the grain, about 1/4-inch thick. Pat the beef dry with paper towels and place in a medium bowl. Add cornstarch and toss until all pieces are well coated. Heat vegetable oil in a large wok or skillet over high heat until shimmering. Working in batches to avoid overcrowding, add the cornstarch-coated beef strips and cook for 2-3 minutes per side until golden brown and crispy on the outside. Remove beef to a plate and set aside. The beef should be crispy but still tender inside.

In the same wok or skillet, reduce heat to medium and add minced garlic and ginger. Stir-fry for 30 seconds until fragrant, being careful not to burn. In a small bowl, whisk together soy sauce, brown sugar, water, hoisin sauce, rice vinegar, sesame oil, and red pepper flakes to create the sauce. Pour this sauce mixture into the wok and bring to a simmer. Cook for 2-3 minutes, stirring constantly, until the sauce thickens slightly and becomes glossy. The sauce should coat the back of a spoon when ready.

Return the crispy beef to the wok with the sauce and toss quickly to coat all pieces evenly. Add the green onion pieces and stir-fry for 1-2 minutes until the green onions are just wilted but still bright green. Remove from heat immediately to prevent overcooking. Transfer to a serving platter and sprinkle with sesame seeds for garnish. Serve immediately over steamed white rice while the beef is still crispy and the sauce is hot. The dish should have a perfect balance of sweet, savory, and slightly spicy flavors with a glossy appearance.