This open-face, "fork and knife" sandwich is piled with slices of Guinness-marinated bison steak, mushrooms and red onion.
- 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). Brush mushroom caps and onion slices with oil, sprinkle with salt and pepper and place in a grilling basket. (No grill basket? See Tip, below.) Place the basket on the grill; place the steaks directly on the grill rack. Grill the steaks and vegetables until browned on the first side, 5 to 7 minutes. Turn everything over and grill until an instant-read thermometer inserted into the center of the steak registers 125°F for medium-rare and the vegetables are soft and lightly browned, about 5 minutes more. Remove from the grill and let rest for 5 to 10 minutes. Lightly grill or toast the bread.
- 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.
- Divide the mushrooms and onions among the pieces of toast and drizzle each with about 1 tablespoon of the reduced sauce. Slice the steak; divide among the sandwiches and drizzle with more sauce. Pour any accumulated juice from the steak over the sandwiches and serve with a fork and knife.