In a medium bowl, combine the oil, beer, garlic, pepper, salt, and mustard. Marinate the steaks overnight in the refrigerator.
In a large skillet, melt the butter and sauté the onions and mushrooms with the paprika until tender and lightly browned; keep warm while cooking the steaks. Remove the steaks from the marinade and broil or grill for about 4 to 5 minutes per side, or to desired doneness. Slice the meat diagonally into ¼-inch pieces. Cut the French bread into four equal pieces, then cut each piece in half horizontally. Divide the onion-mushroom mixture evenly among four pieces of the bread, top with meat, and cover with the remaining bread.