mashed potatoes - fixing undercooked potatoes I decided to do  mashed potatoes. 
I boiled the potatoes (needed 3-5 min more, the were in about 30 min)
I just realised they're undercooked
I  cut the potatoes into smaller pieces (2-3 cm chunks) and put them in the warm pan that I used to boil them


What should I do now. Can it be fixed?
I would suggest microwaving them on to complete the cooking without getting them waterlogged, unless you have a huge volume of potatoes.   They should still be quite hot from the previous cooking, so should not take long to begin cooking in the microwave.

If you don't have a microwave, you could try continuing to boil them, but you don't want to add cold water and have the starches begin to crystallize, so I would try to keep them as warm as reasonably possible while bringing the water to a boil in an electric kettle or another pot.