Hi,
I like cooking big steaks too. My fave at the moment is instead of buying steaks, I cut a single rib off a dry aged rib roast and cook that.
I did a 900 gram one on valentinesnight. Sometimes I finish in the oven, but I follow the Hawksmoor steak rule which is a nice dark almost burned crust on the steak outside.
I had the steak out of the fridge 3 hours before cooking! A big steak will take quite a while for the internal parts to come up to room temperature. If its a big steak and hasn't been out the fridge over an hour forget it. You'll never get the inside cooked without having to finish in the oven.
Heavily seasoned with ELSCo steak rub, and kept on a high heat in the griddle pan. Once a good colour has formed on either side, I keep flipping every minute until the inside gets up to about 50/55 deg C. Then rest on a very warm plate and loosely cover with foil. Alternatively put in the oven set at 50 degrees. This will stop the outside cooling down too much and will continue with the heat delivery into the centre.
I added a knob of gentlemans relish to melt over while resting. The result was superb!
I'll try and post a picture if I can. The inside was perfectly medium rare edge to edge. Frequent flipping is the key to getting an even cook