In a small bowl, mix together 1/2 cup mayo, 1 1/2 Tbsp Dijon mustard, 2 gloves garlic grated, 2 Tbsp chives finely chopped, 1/2 lemon juiced, S&P to taste to make the sauce.
Slice 1 large onion into 1/3inch rounds. In a pan on medium high heat, add 2 Tbsp oil. Once hot, add your onions and cook for 2 minutes or until lightly charred. Then season with salt and pepper and cook for 2 more minutes. Keep warm.
Season steak generously with salt and pepper on both sides. In a frying pan over high heat, heat oil. Next, add your steak and cook for 2 minutes. Then flip and cook for another minute and add butter, garlic, thyme. Baste until your desired doneness. For medium-rare, pull your steak at around 130f. Let rest for 5 minutes.
In the same pan you cooked your steak, toast your bread on both sides.
To assemble, add sauce, tomato and lettuce. Next add your sliced steak and cooked onions. Slice in half and enjoy!
