Grilled Spanish Mustard Beef
Preheat an outdoor grill for high heat, and lightly oil the grate.
Whisk sherry vinegar, olive oil, mustard, paprika, garlic, salt, and pepper together in a large bowl. Place steak in marinade and turn to coat. Marinate at room temperature for 30 minutes.
Cook steak on the preheated grill, turning once, until each side is browned, steak is beginning to firm, and is hot and slightly pink in the center, about 2 minutes per side. An instant-read thermometer inserted into the center should read 140 degrees F (60 degrees C). Transfer steak to a plate and let rest for 5 to 10 minutes before slicing.