Grilled Flank Steak with Mustardy Potato Salad

Fill a large pot with 1 inch of water and fit with a steamer basket. Bring to a boil and cook potatoes until tender, 15 to 18 minutes. Drain and transfer to a large bowl. Toss with chives, mustard, vinegar, and 1 tablespoon oil. Season with salt and pepper. Meanwhile, cook steak. Heat remaining 2 tablespoons oil in a large skillet over medium-high heat. Season steak with coriander and season with salt and pepper. Cook, turning once, until an instant-read thermometer inserted into the thickest part reads 130 º F for medium rare, about 3 to 4 minutes per side. Transfer to a cutting board and let rest 5 minutes, then slice. Serve with potatoes and garnish with chives.