Preheat the oven to 325°. In a small bowl, whisk the mayonnaise with the sour cream, buttermilk, mint, parsley, chives and vinegar and season with salt and pepper. Refrigerate.
Line a large rimmed baking sheet with parchment paper. Arrange the bacon slices on the sheet in a single layer and sprinkle with the brown sugar and cayenne. Bake for about 25 minutes, until caramelized (the bacon will crisp as it cools). Let cool, then cut the bacon into bite-size pieces.
Meanwhile, light a grill or preheat a grill pan. Brush the onions with olive oil and season with salt and pepper. Grill over moderate heat, turning occasionally, until softened and browned, 10 minutes. Separate the onions into rings. Brush the peaches with olive oil and grill over moderately high heat until tender, 2 minutes. Transfer to a plate.
In a large bowl, toss the onions with the peaches and bacon. Add the buttermilk dressing and toss to coat. Serve right away.