Cook the bacon in a large skillet over medium heat, turning once, until very crisp, 7 to 9 minutes. Transfer the bacon to a paper towel-lined plate. Spoon up 1 tablespoon of the remaining drippings and add them to a small measuring cup along with the olive oil.
Whisk together the vinegar, capers, mustard, egg and oil mixture in a large salad bowl. Stir in the chives and parsley. Add the spinach to the bowl and toss to coat. Finely chop the reserved bacon and sprinkle over the top of the salad. Season to taste.
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