3 pounds yellow potatoes, such as Yukon Gold
¾ cup extra virgin olive oil, divided
1 tablespoon fresh rosemary
5 tablespoons fresh Italian flat leaf parsley, divided
1 teaspoon kosher salt or sea salt, divided
¼ cup champagne vinegar
2 tablespoons whole grain mustard
1 tablespoon Dijon mustard
Preheat grill to a medium heat. Bring a large pot of salted water to a boil and drop in the whole skin-on potatoes. Bring back to a boil and cook 8-10 minutes (10 if the potatoes are large). Remove to room temperature to cool. Discard water. The potatoes will not be fully cooked and will finish on the grill.
While the potatoes are cooling, place ½ cup of olive oil, the rosemary, 3 tablespoons of parsley and ½ teaspoon of the salt into a mini food processor (see here) and blend until a liquidy paste forms (it doesn’t need to be perfect). Once the potatoes are cool enough to handle, slice skin-on into thick ¾ inch slices and place on a sheet pan. Spread the rosemary mixture on both sides of the potatoes using all of the mixture.
Gill for 15 minutes turning a few times until fully cooked through and nicely browned. If the oil makes the grill flame up, move them to a higher rack until the flames settle down.
While potatoes are cooking, in a small bowl, mix vinegar, remaining ¼ cup of olive oil, remaining ½ teaspoon salt and both mustards until blended. Chop the remaining parsley and set aside.
As soon as the potatoes come off the grill, toss lightly with the dressing (serve extra dressing on the side) and sprinkle remaining two tablespoons of parsley over the top