What can I use instead of butter for Jamie Oliver's steak recipe?

Jamie Oliver had a steak recipe that was fairly simple:

• Season (salt/pepper/olive oil)


• Dry pan (no oil in the pan) on heat


• Fry a minute+ on each side, flip several times


• When browned, rub the outside with a clove of garlic, some rosemary, and a bit of butter and keep grilling on the pan


• Rest, collect the juices, and use as sauce over the steak.

If I don't want to use the butter (or any diary product) for the above step, what else can I use in that rubbing step instead of the butter? What exactly does the butter rubbing do, what effect would there be if I skip that alltogether, and what can I do to preserve the effect?

He mentioned that you can add butter to the resting juices to sweeten them, but didn't explain what rubbing when grilling does.