Combine ingredients for steak in a gallon zippered bag. Squish around to evenly mix and coat the steak. Seal bag and place in refrigerator for at least 4 hours (up to 24hrs).
In a mixing bowl, combine mayo ingredients. Cover and store in refrigerator until ready to make sandwiches.
Remove steak from refrigerator and place on counter. Preheat grill to high heat (450degF). Once hot, remove steak from marinade (discard marinade) and place on grill. Close grill and cook for 10 minutes. Flip steak and cook for another 5-10 (depending on desired level of doneness). Allow steak to rest at least 10 minutes before slicing into ½ inch thick pieces.
Combine spinach and Italian dressing.
Toast bread (if desired)
Spread both sides of bread with mayo mixture. Then layer on portion of steak, slice of swiss cheese, portion of spinach, and red onions. Place top of bread on and press down slightly. Secure with long toothpicks, if desired.