In a large pot fit with steamer/pasta basket fill with enough cold water to just touch the bottom of the basket. Season with kosher salt. add washed potatoes, cover and bring to a boil, steaming potatoes until just barely fork tender. Remove and allow potatoes to steam dry for 10 minutes, or until just cooled enough to handle.
Heat stock, vinegar, mustard, kosher salt and black pepper until hot or very warm; set aside.
Using grandma’s old plastic handled steak knife (or any dull serrated knife), peel potatoes using a gentle scraping action, so as to not remove too much of the potato or ruin the texture.
In the same order, cut potatoes into roughly 3/4” squares and place in a large stainless steel mixing bowl. Pour stock mixture over the potatoes and gently swirl the bowl to coat the potatoes. Set aside for 15-30 minutes until liquid is absorbed or nearly so.
Brunoise a red onion (small dice), thin-slice the cornichons, mince the dill and thin-slice the bacon. Add these ingredients and drizzle in the extra virgin olive oil and cornichon juice. Gently fold the ingredients into the absorbed cooled potatoes. Adjust for more kosher salt, black pepper, white wine vinegar and whole grain mustard.
Plate and serve. Best served warm or room temperature.