Combine all the marinade ingredients into a sturdy zip-top plastic bag or a glass bowl. Mix well and add in the steak. It should marinate for at least 2 hours or overnight.Just make sure you bring the steak to room temperature before you cook it and let it rest at least 10 minutes before you slice it.
Heat your pan to medium high heat. Brush your onion slices on both sides with olive oil and season with salt and pepper. Place onion slices in hot pan and cook for about 5 minutes per side, keep a check on heat so they don’t burn. When onions are to your liking, remove from pan and set aside.
Add more oil if you need to. Remove steak from marinade and bring to room temperature. Season with a little salt and pepper. Sear steak in hot pan for the temperature you like your steak. I like a medium rare steak so I will grill it about 2 ½ minutes per side. Place steak on a cutting board, cover it with foil and let it rest 5 to 10 minutes. Slice steak across the grain.
While steak is resting toast buns in the hot pan you cooked your steak in or under a broiler, your choice.
Spread mayonnaise on both sides of the bun. Starting with the bottom half place a couple lettuce leaves, then slices of steak followed by slice of tomato and finally the top half of the bun.
Slice sandwich in half if you like and eat with a side of dill pickles. Let's Eat!!