Preparation

• Combine onion and red wine vinegar in a measuring cup or other tall vessel. Place a glass or small plate on top of onion to submerge. Let sit for at least 30 minutes.

• Meanwhile, fill a large pot with 2 inches of water and fit with a steamer basket. Add potatoes and sprinkle with 2 Tbs. salt. Bring water to a boil and cover pot. Reduce heat to maintain a vigorous simmer and steam until potatoes are tender when pierced with the tip of a knife, 10 to 15 minutes. Transfer to a large bowl.
• In a medium skillet over medium heat, cook the bacon, stirring occasionally, until crispy and fat has rendered, about 10 minutes. Remove with a slotted spoon to a paper towel-lined plate and drain the bacon fat from the skillet. Drain the onion from the vinegar, reserving the vinegar. Add the vinegar to the skillet and cook until slightly reduced, about 2 minutes. Add the mustard and olive oil and continue to cook, swirling the skillet or whisking to emulsify.
• Pour the dressing over the potatoes and fold to combine. Add the celery, parsley, dill, and reserved red onion, and season generously with salt and pepper. Scatter the bacon and dollop the crème fraîche over the top. Serve immediately or at room temperature.