Mostly all-at-once. I hate when someone teaches me to do something, then says it was all wrong. Why did they waste my time on the wrong thing?
But sometimes I am not strong enough, or controlled enough, to do something "right" at first.
It's fine to teach a foundation skill first, then add more to it, as long as I know that is what is being done at the start.
As a very simple example, take swizzles. Done right, with deep knee bends at the start, straightening as the feet come together, deep inside edges, and outwards, then inwards pressure created by properly alignment, they feel nearly effortless. But that is a lot of things to remember at once, and not everyone is strong enough for deep knee bends, and the bend/straighten exercise requires timing. So you have to learn it in pieces - but I think it should all be demoed, right, at the start.