Once the potatoes are overcooked, there is little you can do to give them structure again.
Your best bet would be to re-purpose the dish, making them into potato pancakes, a home fry, or some other type of dish, depending on how much structure is left.
In the future, you can minimize the chances of mushy breakdown by:
- Use a waxy potato (such as the US varieties Red Bliss or fingerling). These hold their structure better than starchy potatoes like Russets.
- Start the par-cook in cold water, and remove the potatoes as soon as it is comes to the boil. This will still shorten your roasting time, but minimize the likelihood of severely overcooking.
- Add a small amount of vinegar to the par-cooking liquid, which will help the pectin retain structure.
- Depending on the variety of potato you are cooking, don't cut them up into small chunks. It is much easier to overcook small chunks than larger ones.
- Don't par-cook. You will require longer for roasting, but it is slower and more controlled, and less likely to overcook.