Boil the cubes of sweet potato and potato in a pot for 15-20 minutes or until soft but not mushy, then drain in a colander and let cool slightly. In the meantime, fry the bacon in a small frypan with the oil, drain on paper, then chop into bite sized pieces when cool.
In a small bowl, mix together the mayonnaise, mustard, vinegar, turmeric, chives, salt and pepper. Transfer to a large bowl and mix in bacon bits, spring onion and red onion. Add the potato and sweet potato to the mayonnaise mixture.
Garnish with extra chives and spring onion if desired.