Place potatoes on a foil-lined rimmed baking sheet. In a small bowl, whisk oil, mustard and vinegar; season generously with salt and pepper. Brush some vinaigrette over the potatoes. Tuck bay leaves between the slices of each potato, spacing evenly apart. Roast at 425 degrees , 30 minutes; brush with more vinaigrette. Roast until tender, about 30 minutes more. Broil until browned, 1 to 3 minutes. Season; brush with remaining vinaigrette.
