There's a 'cheat' I learned many years ago - by accident.
If you parboil potatoes, the starches change in such a way as to make the outside fluffy, yet the inside not collapse - for if you're making roast potatoes, or twice-cooked chips [fries].
With some experimentation on parboiling, I arrived at a method by which you don't let them even parboil, you pull the heat early, then let them cool naturally, still in the same water & pan.
I use a glass-lidded heavy saucepan for this, so I can watch for the 'turn'.
Start with regular 'winter crop' [starchy] potatoes; peeled & cut into your final sizes.
In a heavy saucepan, starting with cold water, bring the pot up to the boil… but don't let it quite reach a full boil.
At one point you will see the potatoes go 'glassy', slightly translucent. As soon as this seems to be through the entire pot, switch it off & allow to cool naturally.
This precise point may need some experimentation - too soon & you get potato-shaped bullets which will never soften, too late & you needn't have bothered, they will continue to cook like 'regular potatoes' & turn to mush in your stew. If you are fearful you stopped late, take the lid off, if you feel you stopped early, leave it on. The difference in heat retention can just make the difference.
One thing - don't force-cool them by putting the pan or the potatoes in cold water… you will get black-edged potatoes.
There is a chemical transformation that happens here that will then mean the potatoes will never go fully soft. You can then re-cook them for hour on hour in a slow cooker, low on a hob, or in the oven & they will never turn to mush.
This all has a firm scientific basis - not one bit of which do I understand. I prefer to think of it as 'voodoo magic' - do it this way and it works;)
