Place ground beef into a large bowl. Add 1/3 cup of stout beer, salt, Worcestershire sauce, onion powder, garlic powder, and black pepper to meat. Combine well, but do not over mix. Form into 6-7 oblong patties, a little bigger that the length and width of marbled rye slices. Place patties onto grill and cook for 5-6 minutes per side. Toward the end of cooking, add 2 Swiss cheese slices to each patty and cook until cheese has melted. Remove from heat and arrange onto toasted rye bread. Top with sauteed onions and remaining bread halves.