01 -
Fire up your grill to 425°F with indirect heat. Toss ¼ cup of olive oil and seasoning into a food processor, blend until it's smooth, about 30 seconds. Make sure to cover your steak with this mixture and let it sit at room temp for 20 minutes.
02 -
Coat onion slices with the leftover olive oil. Place them over direct heat and cook for 3-4 minutes, flipping as needed. Move to a cooler area on the grill if they cook too much too fast.
03 -
Cook steak directly on grill grates for about 2 minutes per side. Shift it to the cooler side, cover, and let it cook another 5 minutes for medium. Set it aside for 5 minutes to rest before cutting thinly across the grain.
04 -
While steak is cooling off, mix mayo, horseradish, chives, black pepper, and salt together until creamy.
05 -
Spread horseradish mayo on both bun halves, then add onions, lettuce, and slices of steak for stacking.