Place flat-iron steaks in a resealable plastic bag. Combine marinade ingredients and pour over steaks. Make sure steaks are well submerged. Seal bag and place into refrigerator for 4-8 hours.
To prepare au jus, combine beef base (I used Better than Bouillon, roast beef base), with 2 cups warm water. Add to a large saucepan. Add ale, sliced onion, and remaining ingredients for jus. Allow mixture to come to a near boil, reduce heat and simmer for 35-45 minutes. If sauce begins to reduce too much, add 1/2 to 1 cup of water until it reaches the right consistency.
Preheat grill for medium-high heat (375 degrees F). Remove steaks from bag and place them onto grill. Cook for 8-10 minutes per side, depending on thickness. Once steaks have cooked to desired doneness, remove from grill, tent with foil and allow them to rest for 5-7 minutes before carving. Slice against the grain.
To assemble, apply mayonnaise (or mustard) first, then lettuce (optional), two slices of Swiss cheese (or crumbled cheese) per sandwich. Next, place desired amount of simmered onions from au jus, onto bread. Finally add steak slices. Using a ladle, scoop (or strain) au jus , minus onion into a small bowl. Dip sandwich into jus and enjoy.