Preheat the grill to high. Place potatoes in a large pot with the onion and cover with cold water. Cook, on the grates of the grill, or on a burner, until tender. Drain, discard the onion, and cut the potatoes into cubes when cool enough to handle. Place the potatoes in a large bowl and cover to keep warm.
Place a large saute pan on the grates of the grill. Add the bacon and cook until crisp. Remove the bacon with a slotted spoon and drain on a paper towel-lined plate. Add the onions to the rendered bacon fat and cook until soft, about 3 to 4 minutes. Carefully add the vinegar and mustard seeds and cook for 2 more minutes. Whisk in the canola oil and season, to taste, with salt and pepper. Add the hot dressing to the potatoes and toss gently to coat. Fold in the green onions and parsley. Season again with salt and pepper, to taste.
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