Put the potatoes into a saucepan and cover with water.
Add 1 teaspoon of kosher salt.
Bring water to a boil, cover and continue boiling until the potatoes are just starting to get tender. Do not overcook them. Start checking them after 10 minutes.
Remove from heat, drain, place in a large bowl, and let cool.
Add the chopped eggs, peas and onion. Fold to combine. Cover and refrigerate for 30 minutes.
Whisk together 2/3 cups of the mayo, the sour cream and mustard in a medium bowl.
Fold into the potato mixture. Add a bit more mayonnaise if too dry.
Season with salt and pepper.