Whisk together mayonnaise, garlic, and both mustards in a bowl, season with salt and pepper. Cover aioli and refrigerate at least 30 minutes and up to 1 day.

Meanwhile, put potatoes in a pot, cover with cold water by 2 inches, and add 2 Tbsp salt. Bring to a boil and cook 15 to 20 minutes {a skewer inserted into a potato should meet some resistance}. Drain and let cool slightly

Heat grill to medium for direct grilling. {Gas grill: Turn on all burners. Charcoal grill: Rake coals into an even layer.]

Put potatoes in a large bowl, toss with aioli, and season with salt. Grill until golden brown on all sides, about 8 minutes.

Transfer potatoes to a platter, sprinkle with herbs, and season with salt and pepper.