Place the cubed potatoes in a large pot and cover with cold water by about an inch. Add 1 teaspoon of salt to the water. Bring to a boil over high heat, then reduce heat and simmer for 15-20 minutes, or until the potatoes are fork-tender but not mushy. Drain thoroughly and let cool slightly in the colander for about 10 minutes to allow steam to escape.

While the potatoes are cooking, bring a small pot of water to a boil. Add the thawed frozen peas and cook for 1 minute. Immediately drain and transfer the peas to a bowl of ice water to stop the cooking and preserve their bright green color. Drain well after 2-3 minutes.

Finely dice the red onion. Chop the fresh dill, dividing it into two equal portions – one for the dressing and one for garnish/final mix-in.

In a large bowl (the one you'll use for the salad), whisk together the mayonnaise, Dijon mustard, apple cider vinegar, 0.75 tsp salt, 0.25 tsp black pepper, and half of the chopped fresh dill until smooth and well combined.

Add the slightly cooled, drained potatoes, blanched peas, and diced red onion to the bowl with the dressing. Gently fold everything together with a large spatula until the potatoes and vegetables are evenly coated. Be careful not to mash the potatoes.

Cover the bowl and refrigerate for at least 1 hour, or preferably 2-3 hours, to allow the flavors to meld. Before serving, stir gently and adjust seasoning if needed. Garnish with the remaining fresh dill.