Mongolian Beef

In a small saucepan over medium heat, heat 1 tablespoon vegetable oil. Add garlic and ginger and cook until fragrant, 2 minutes. Add soy sauce, water, and brown sugar and stir until dissolved. Bring to a boil, then reduce heat and simmer until reduced by half, 10 to 12 minutes. Meanwhile, heat 1/4 cup vegetable oil over medium heat in a large skillet. In a large bowl, toss flank steak with corn starch until fully coated. Add steak to skillet and sear until crispy, 3 to 4 minutes per side. Drain fat. Add sauce and scallion quarters to skillet and toss until combined, then simmer a few minutes more. Serve steak in lettuce cups and garnish with chopped scallions.