Boil the potatoes: In a large pot, add potatoes and enough water to cover by 1-inch. Season generously with salt. Bring to a boil over high heat. Once boiling, reduce heat to medium-high and cook for 14-18 min, until potatoes are fork-tender. Strain and set aside to cool.
Boil the eggs: Bring a medium pot of water to a boil. When it reaches a rolling boil, lower eggs into the water gently. Boil on high for 9 minutes (for a medium-ish yolk) Strain and rinse with cold water. Peel and roughly chop.
Mix the dressing: Meanwhile, in a medium bowl, mix mayonnaise, both mustards, 1 tbsp olive oil and cider vinegar. Season with salt and pepper.
Chop potatoes: When the potatoes have cooled enough to handle, roughly chop into 1 or 2 inch peices. Add to a large bowl (or put them straight into a large resealable container).
Finish Herby Potato Salad: To the bowl with potatoes, add boiled eggs, green onions, parsley, dill (reserving some herbs, eggs and onions for garnish later!) Throw all of the capers and pickles in. With a large spoon or spatula, mix to combine. Don't mash them too roughly, but don't be too gentle either. The goal is slightly roughed up potatoes that still have their structure. Season the whole thing with salt and pepper.
Make Ahead: Refridgerate until ready to serve, up to 2-3 days. To serve, sprinkle over reserved herbs, green onions and eggs. Finish with a hit of flaky salt and freshly cracked pepper if desired.