Preheat the oven to 425 degrees F. Combine 1/2 teaspoon each coriander, cumin and salt in a small bowl. Sprinkle the spice mixture all over the steak and rub into the meat. Heat a large ovenproof skillet over medium-high heat; add 2 tablespoons olive oil. Add the steak and cook until browned, about 1 minute per side.
Scatter the potatoes around the steak and stir to coat in the oil; season with salt and pepper. Transfer the skillet to the oven and roast until the potatoes are tender and a thermometer inserted sideways into the center of the steak registers 125 degrees F for medium rare, about 20 minutes. Remove the steak to a cutting board; let rest 10 minutes. Return the potatoes to the oven and roast until crisp, about 5 more minutes.
Combine the cherries, 2 tablespoons parsley, 1 sliced scallion, the vinegar and the remaining 1 tablespoon olive oil and 1/2 teaspoon each coriander and cumin in a bowl. Season with salt and pepper.
Transfer the potatoes to a large bowl. Add the remaining 2 sliced scallions and 2 tablespoons parsley; toss. Slice the steak against the grain; top with the cherry tapenade. Serve with the potatoes.