Ensure the potatoes are of equal sizes.
Boil them in salted water until tender. This typically takes 20 minutes.
Drain the water and let the potatoes cool. The quickest way to cool them is in a dish where they don't overlap, in the fridge/freezer.
Grate the onions into a bowl along with curry, salt, and pepper. Add mayonnaise, sour cream, and lemon juice, and mix the dressing well.
Mix the potatoes with the dressing and let them sit for at least 15 minutes before serving.
Just before serving, add the chopped herbs, preferably in large quantities.
The potato salad goes well with most grilled meats.