For the mustard aioli: Whisk the mayonnaise, Dijon mustard, whole grain mustard, honey and garlic and sprinkle with salt and pepper Cover the bowl and refrigerate for at least 30 minutes and up to 1 day.
Meanwhile, for the potatoes: Put the potatoes in a pot and cover with cold water by 2 inches. Add the salt and bring to a boil over high heat. Cook the potatoes until a skewer inserted into the center of each potato meets with some resistance, 12 to 15 minutes. Drain well and cool slightly.
Preheat the grill to medium for direct heat. Cut the potatoes in half, toss with some of the mustard aioli and sprinkle with salt. Place the potatoes cut-side down on the grill and grill until lightly golden brown, about 3 minutes. Flip the potatoes and grill until golden brown and just cooked through, about 3 minutes. Transfer the potatoes to a platter and sprinkle with the parsley, chives, tarragon, salt and pepper. Serve the potatoes with the remaining mustard aioli.