Combine beer, sweet onion, soy sauce, molasses, rosemary, garlic and Worcestershire sauce in a large sealable plastic bag. Add steaks. Seal the bag, shake and turn it, and refrigerate, flipping over and shaking the bag from time to time, for 12 to 18 hours.
Preheat a grill to medium. Remove the steaks from the marinade and pat dry. Reserve 1 cup marinade (discard the rest). Grill the steaks until browned on the first side, 5 to 7 minutes. Turn over and grill until an instant-read thermometer inserted into the center of the steak registers 125°F for medium-rare. Remove from the grill and let rest for 5 to 10 minutes.
Meanwhile, place the reserved 1 cup marinade in a small saucepan and bring to a simmer over medium-high heat. Boil until reduced by about half, 7 to 10 minutes.
Drizzle each steak with about 1 tablespoon of the reduced sauce and serve.