Place the potatoes in a large pot of cold water, add 2 tablespoons of salt and cook until potatoes are tender, but still firm, 12 to 15 minutes. Drain, let cool then cut each potato into rounds and wedges. Potatoes can be made up to 2 days in advance and refrigerated and cut just before grilling.
While the potatoes are cooking, combine the zest, juice, garlic, mustard, rosemary and salt and pepper in a blender and blend until smooth. With the motor running, slowly add the extra-virgin olive oil and blend until emulsified. Can be made 1 day in advance and refrigerated. Bring to room temperature before using.
Heat the grill to high.
Brush the potatoes on both sides with the canola oil and season with salt and pepper. Place the potatoes on the grill, cut side down and cook until golden brown, 4 to 5 minutes. Turn the potatoes over, and continue grilling until just cooked through, about 5 minutes longer. Remove the potatoes and cut each half in half lengthwise and then crosswise into 1-inch pieces. Transfer to a platter and immediately drizzle with the vinaigrette and toss to combine. Garnish with parsley sprigs.