Beer-Marinated Grilled Flank Steak

1. In a large bowl, mix onion, mustard, soy sauce, rosemary, and garlic. Stir in beer.2. Transfer steak to a large resealable bag or container and pour marinade over steak, seal bag, and turn to coat. Refrigerate overnight, turning occasionally.3. Heat a grill to medium-high and brush grates with oil. Remove steak from marinade and scrape off any marinade; discard marinade. Season steak with salt and pepper. Grill steak for about 5 to 6 minutes per side, turning once, for medium-rare doneness, or 7 to 8 minutes per side for medium doneness. Test with an instant-read thermometer; steak should register 145°F in the thickest part for medium. Let rest for 5 minutes, then thinly slice across the grain.