When the potatoes remain to cook for another 10 minutes, drain some of the water into a small saucepan, add 1 tsp sugar and 1 tsp lemon juice (I think lemon does not matter, but I did as in the recipe).
In this water we put ... Yes, peel and core the apples, cut into slices! Since the water is already hot (water has also been drained from the potatoes), it boils very quickly. From the moment of boiling, cook for 5 minutes.
P. S. Potatoes, meanwhile, continue to cook.