Grilled Lamb With Plum Mostarda and Blistered Shishitos

Name: Grilled Lamb With Plum Mostarda and Blistered Shishitos
Ingredients: 5 tablespoons olive oil, divided; 1 shallot, chopped; 2 tablespoons yellow mustard seeds; 2 tablespoons granulated sugar; 2 tablespoons red wine vinegar; 1 pound plums, pitted and chopped; 5 ¼ teaspoons kosher salt, divided; 1 5-lb. boneless leg of lamb, butterflied and trimmed; 2 tablespoons finely chopped fresh thyme or oregano; 2 teaspoons freshly ground black pepper, plus more for serving; 12 ounces shishito peppers; 1 teaspoon flaky sea salt