What can I use instead of butter for Jamie Oliver's steak recipe?
The butter is there for flavor, mouth feel, and to add some body to the sauce you make. I'd replace the butter with.... nothing at all. Many of the flavor compounds in garlic and rosemary are fat soluble and fat is not exactly in short supply when it comes to steak, so you should be able to run those on directly and still get some flavor. Alternatively you could steep some olive oil with the garlic and rosemary and brush a bit on, but I would just simmer them in the juices for a minute instead and then spoon them on the cooked steak. 

I have a few comment's on Jamie's recipe:


Do not use olive oil when frying steak, it has too low a smoke point and may turn bitter. Use vegetable oil (canola/rapeseed, corn, peanut, sunflower, safflower, or blend) instead.
Do not pepper the steak before frying as pepper will burn and turn bitter. Add fresh pepper just after you remove it from the pan to rest. If you want a pepper flavor inside the steak use a marinade. Salting before is fine
The oil is not a seasoning, that's the salt and pepper. The oil's purpose is to help conduct the heat from the pan to the steak until the steak's juices come out
There flip repeatedly method works but it isn't the only way to do it, I prefer to flip once after about 65% of the cooking time has elapsed. Doing a 65-35 rather than a 50-50 flip will make your steak evenly pink in the middle and means you can turn your attention to other things. I also think you get a better sear when using a pan. I use the 1 minute flip over charcoal though as it keeps the steak from getting too crispy