[Hayarotle]

It's an useful technique and I've already used it many times, but what do you do if there isn't enough genuine positive feedback for you to give to the person to make the constructive feedback more palatable?

I find it to be a bit of a crutch. I prefer instead to make the constructive feedback itself palatable and not "shit". If it's an opportunity for growth then I frame it as that. If it's just factual feedback without any harmful consequences for the person's career or employment, I make that clear. In those situations we should be able to give harsh feedback without making it unpalatable.

If it does have harmful consequences for the person, then it should be made clear and we should provide help to them (and frame it as just that, as help rather than as an accusation). If the consequences are immediate, then it isn't just feedback and should be dealt with appropriately.