Add the cashews to a bowl and cover with boiling water. Let soak for 15 minutes.
Drain the cashews and blend with dill pickle brine, Dijon mustard, garlic, salt, and water until smooth and creamy.
Toss the sliced red onion with lemon juice in a small bowl and set aside.
Cut the potatoes into chunks and add to a large pot with salted water. Bring to a boil, reduce to a simmer, and cook for 6 to 10 minutes until fork tender.
Drain the potatoes and transfer to a bowl. Toss with dill pickle brine and salt while still warm, then cool completely.
Add the eggs to a pot and cover with water. Bring to a boil, remove from heat, cover, and let sit for 10 minutes before transferring to ice water.
Peel and chop the eggs.
Add the cooled potatoes, eggs, diced pickles, red onion mixture, fresh dill, and a few spoonfuls of dressing to a large bowl.
Stir gently until combined, adding more dressing as needed.
Garnish with extra pickles, red onion, and dill before serving.