Preheat the oven to 450°F convection. In the oven, preheat a large cast-iron pan for 20 minutes. When you remove it from the oven, the handle will be very hot, so leave a pot holder or dish towel on it for safety.
Season both sides of the steak generously with salt and freshly ground pepper.
Add the grapeseed oil to the preheated pan over high heat and then sear the steak, laying it down away from you to prevent splattering. Cook the first side until the surface is a rich golden brown. Do not touch until it’s ready to turn, about 1 ½ minutes. Flip and cook the second side for an additional 1 ½ minutes.
Transfer the steaks from the pan to a plate and reserve.
Add the onions to the pan, season with salt and freshly ground pepper and cook for about 30 seconds to begin sweating.
Place the steaks on top of the onions and place the pan in the oven for about 10 to 12 minutes, for medium rare. The steaks will be between 128 and 130 degrees, but will carry-over cook to about 130 to 135 degrees.
Remove from the pan from the oven, transfer the steaks to a cutting board and allow them to rest for 5 to 10 minutes. Slice the steak against the grain and serve on top of the onions.