In a medium bowl, combine the beef slices, 1 tablespoon of the oil, the cornstarch, salt and a few grinds of pepper. Toss together; set aside.
Heat a wok or skillet over medium-high heat until a drop of water evaporates on contact. Add 1 1/2 tablespoons of the oil and heat until shimmering. Add half the beef and cook, stirring, until the beef is browned, about a minute. Add half the garlic, half the ginger and half the fish sauce and continue cooking until the beef is cooked through, about two minutes. Transfer to a bowl. Repeat with the remaining oil, beef, garlic, ginger and fish sauce.
On each serving plate, mound vermicelli and lettuce, then spoon the beef over them. Sprinkle with peanuts and cilantro. Serve.