Place the steak, beer or wine, minced garlic, salt, and pepper in a large “Ziploc” bag and marinate for several hours or overnight in the refrigerator.
Sauté onions in olive oil until lightly browned. Remove steak from marinade. Add marinade and mushrooms to onions and continue to sauté over medium heat to reduce marinade by half. Broil steak on grill or in oven for approx. 5–7 minutes on each side, or to medium-rare doneness depending on thickness. While steak is broiling, toast French bread. Mix sour cream, horseradish, and Worcestershire sauce together in a serving bowl.
Thinly slice the steak on the diagonal across the grain. Arrange desired amount of steak on 2 slices of toasted bread. Top with the hot onion/mushroom mixture and finish with sour cream horseradish sauce.