at the steak completely dry with paper towels, since a wet surface won't crust properly in the pan. Rub the spice blend generously over both sides, pressing it in with your fingers so it really sticks. Let it sit at room temperature for about 5 minutes so the spices bloom slightly.
Heat the oil in your skillet over high heat until it's shimmering and almost smoking. Lay the steak in carefully and don't touch it for 3-4 minutes per side, depending on thickness. You're looking for a deep brown crust and an internal temperature of about 130-135°F for medium-rare.
Move the steak to a cutting board and let it rest for 5 minutes before slicing, which keeps all those juices inside instead of on your board. Slice it thinly against the grain for the most tender bite. Skipping this rest is the easiest way to end up with a dry, tough salad topper.
While the steak rests, whisk together the mayonnaise, Greek yogurt, minced garlic, and lemon juice in a small bowl. Season with a pinch of salt and pepper, then taste and adjust the tang or creaminess as you like. This dressing should be thick enough to coat a spoon but still pourable.
Spread the lettuce out on a large plate or in a wide bowl as your base layer. Scatter the cucumber and tomato halves over the greens, then fan the sliced steak across the top. Finish with crumbled feta and a generous drizzle of the dressing, then serve right away while the steak is still warm.