Step 1

Place potatoes in a medium saucepan, cover with 3 qt. water and ½ cup kosher salt, and bring to a simmer over medium-high heat. Cook, uncovered, until potatoes are tender when pierced with a fork, 12–14 minutes. Drain potatoes and transfer to a plate. Let sit until cool enough to handle.

Step 2

Meanwhile, cut steak in half crosswise. Season all over with kosher salt. Transfer steak to a shallow bowl and add 2 Tbsp. oil and 2 Tbsp. harissa. Using tongs, turn to coat. Let steak sit at least 15 minutes and up to 24 hours.

Step 3

Whisk together vinegar and 3 Tbsp. oil in a medium bowl. Gradually whisk in 2 Tbsp. harissa. Taste the dressing. If it’s pungent and strong, stop there; if you feel like it could take a bit more heat and flavor, add remaining 2 Tbsp. harissa. Add potatoes to dressing, smashing them lightly with your hands to expose some of the flesh so it can soak up some of the vinaigrette.

Step 4

Heat remaining 2 Tbsp. oil in a medium skillet over medium-high. Cook steak in a single layer, undisturbed, until well charred, 3–4 minutes. Turn steak and continue to cook until other side is charred, 2–3 minutes longer. Transfer steak to a cutting board. Let rest for a few minutes, then thinly slice against the grain.

Step 5

If packing for lunch, divide potatoes and harissa dressing between 2 airtight containers. Layer sliced steak over potatoes. Divide greens and radishes between containers. Season with sea salt and a few cranks of pepper. Nestle 1 lemon wedge into each. Cover and chill until lunchtime.

Step 6

Let container sit at room temperature for 15 minutes before eating so oil loosens up and meat isn’t fridge-cold. Squeeze lemon wedge over, reseal container, and shake to coat everything in dressing. Transfer steak salad to a bowl if desired.

Step 7

Do Ahead: Steak salad can be assembled 3 days ahead. Keep chilled.