Prepare fresh cucumber: Stir sugar, cucumber, 1/2 cup vinegar, dill, and 1/2 teaspoon salt together in a bowl and set aside.

Boil potatoes: In a large pot, combine potatoes, remaining vinegar, 1 tablespoon salt, and 8 cups water. Bring to a boil, then reduce heat and simmer for 10-15 minutes, or until potatoes are just tender.

Drain potatoes: Drain the potatoes and return them to the pot.

Fry bacon: In a large skillet, fry bacon over medium heat until brown and crisp, about 8 minutes. Transfer bacon to a paper towel-lined plate, reserving 2 tablespoons of bacon grease in the skillet.

Sauté onion: Add onion and olive oil to the skillet and cook over medium heat until softened and beginning to brown, about 4 minutes.

Drain cucumbers and reserve juice.

Make dressing: Whisk cucumber juice, mustard, and 1 teaspoon pepper into the skillet and bring to a simmer.

Combine dressing and potatoes: Pour the dressing over the warm potatoes and stir to combine. Let it sit for about 10 minutes to allow the potatoes to absorb the dressing.

Assemble salad: Stir parsley, drained cucumbers, and bacon into the potatoes to combine. Season with salt and pepper to taste.

Transfer salad to a serving bowl and serve. The salad can be refrigerated in an airtight container for up to 1 day. Bring to room temperature before serving.