Parsley, mustard, and vinegar are the base for the flavorful sauce that tops the steak. Make a double batch, and use it on the green beans too.
For the salsa verde: In a small bowl, mix together parsley, capers, shallot, vinegar, mustard, and 1 tablespoon water; season with salt and pepper.
For the steak: In a small skillet, heat 2 teaspoons of the oil over medium-high. Season steak generously with salt and pepper. Cook, turning once, until well browned, 2 to 3 minutes per side for medium-rare. Remove steak from pan; let rest 5 minutes. Serve with sauce.
For the green beans: Meanwhile, in a small saucepan, bring 1 inch salted water to boil over high heat. Add green beans; cover. Cook until crisp-tender, about 5 minutes. Drain beans, place on a plate, and drizzle with remaining teaspoon oil. Season with salt and pepper.