1. Whisk together the mayonnaise, garlic, and both mustards in a small bowl; season with salt and pepper. Cover the aioli and refrigerate for at least 30 minutes and up to 1 day. 2. Put the potatoes in a pot, cover with cold water by 2 inches, and add 2 tablespoons salt. Bring to a boil over high heat and cook until a skewer inserted into the center of a potato meets some resistance, 15 to 20 minutes. Drain well and let cool slightly. 3. Heat your grill to medium for direct grilling. 4. Put the potatoes in a large bowl, toss with the aioli, and season with salt. Grill until golden brown on all sides, about 8 minutes. 5. Transfer the potatoes to a platter, sprinkle with the fresh herbs, and season with salt and pepper.