Combine 1 Tbsp. sesame oil, 1 Tbsp. soy sauce, garlic and 1 Tbsp. minced lemongrass in a food processor and blend until smooth. Pat the steak dry with a paper towel; season with salt and pepper.
Place the meat in a large baking dish and pour the marinade over the top, turning evenly to coat. Let sit for 15 minutes.
Meanwhile, mix together the rest of the lemongrass, 1 Tbsp. sesame oil, 1 Tbsp. soy sauce and rice wine vinegar. Using a vegetable peeler, slice the cucumber into thin ribbons and cut the bell pepper into strips.
In a large wok or frying pan over medium-high flame, heat 1 Tbsp. cooking oil until hot but not smoking. Add the steak and cook, stirring occasionally, until lightly browned but not cooked through, 2-3 minutes per side. Transfer the meat to a cutting board and let rest for 5 minutes. Slice the steak against the grain into ¼- inch strips.
Divide the salad mix between two plates and top with cucumber, bell pepper, steak strips and scallion. Pour dressing over salad and serve.