Remove the beef steak from the refrigerator half an hour before cooking. Pat it dry, sprinkle with black pepper and cook it in a hot pan with a splash of olive oil for two minutes per side. Let the meat rest for five minutes under foil before slicing it thinly, so the juices stay in the meat.
Meanwhile, make the dressing: whisk the lime juice with the fish sauce, the remaining olive oil and the finely minced red chili. Fish sauce provides the salty, savory base that gives this salad its Thai character.
Slice the cucumber into half-moons and the red onion into very thin slices. Toss them with the little gem lettuce and beef slices in a bowl and pour the dressing over. Finish generously with cilantro and mint. Plenty of protein, few carbs and a freshness that handles summer heat wonderfully.