Leg of Lamb with Pistachio Stuffing and Potatoes

Name: Leg of Lamb with Pistachio Stuffing and Potatoes
Ingredients: 1 bunch scallions, light-and dark-green parts only, cut into 2-inch pieces; 1 bunch curly parsley, thick stems removed (2 lightly packed cups), plus more for serving; 1 tablespoon finely grated lemon zest (from 2 lemons), plus 2 tablespoons fresh juice; ½ cup salted roasted pistachios; 4 tablespoons extra-virgin olive oil, divided; Kosher salt and freshly ground pepper; 2 tablespoons coriander seeds; 2 tablespoons cumin seeds; 2 tablespoons mustard seeds; 3 tablespoons minced garlic (from 1 head); 1 boneless leg of lamb (2 ½ to 3 pounds); 2 ½ pounds unpeeled russet or Yukon Gold potatoes, cut into ¾-inch-thick rounds; 1 tablespoon chopped fresh rosemary leaves, plus a few whole sprigs; Flaky sea salt, such as Jacobsen, for serving (optional)