For all I know there is a special starter strip made for 1/2" insulation, that both holds the foam board and gives you the needed lip for the first course. I have never seen it, but it would be worth looking in to. Failing that, rip a piece of 1/2" plywood and add that strip to the wood siding to back up the ordinary starter strip so it's not nailed into foam. That ought to keep the line stronger and straighter than if nailed into the foam (which will compress). And remember to "hang" the siding, don't nail it tight.
Casey