In a large pot, cover the halved baby potatoes with water and season generously with salt. Bring the water to a boil and cook until the potatoes are easily pierced with a knife, about 15 to 20 minutes. Drain and let cool slightly before transferring to a large serving bowl.

In a large skillet over medium heat, cook the bacon until crispy, about 8 minutes. Reserve about 2 tablespoons of bacon fat in the pan, and transfer the bacon to a paper towel-lined plate to drain.

Add the finely chopped red onion to the skillet with the reserved bacon fat and cook over medium heat until starting to soften, about 3 minutes.

Whisk in the apple cider vinegar, water, olive oil, Dijon mustard, and sugar. Bring the mixture to a simmer, then turn off the heat and season with salt and pepper to taste.

Stir in the sliced green onions and gradually pour the warm dressing mixture over the cooked potatoes. Crumble the crispy bacon over the potatoes and toss everything together gently to combine. Serve the salad warm.