Prepare the sesame mayonnaise by mixing mayo with the sesame oil and lime juice. Season with salt and pepper and set aside.
Mix all ingredients for the beef marinade and pour into a plastic bag. Add the beef and marinate for 15 minutes or more at room temperature, or overnight in the fridge.
Chop all vegetables for the salad, except the scallions, into bite-sized pieces. Divide between two plates.
Pat the meat dry on both sides with paper towels. On high heat, sear for a minute or two on each side, and then reduce heat to medium low, cooking until beef is medium, and then transfer to a cutting board.
Fry the scallions for a minute in the same pan.
Slice the meat, across the grain, into thin slices. Place beef and scallions on top of the vegetables.
Top with sesame seeds and serve with a dollop of sesame mayonnaise on the side.