Scallion Potato Pancakes

Name: Scallion Potato Pancakes
Ingredients: 2 bunches scallions (about 7 ounces); 2 1/2 pounds russet potatoes, peeled; 1 small (6-ounce) yellow onion , peeled and trimmed (root end intact); 2 large eggs, lightly beaten; 1/2 cup potato starch (about 2 ounces); 1/2 cup bleached cake flour (about 1 7/8 ounces); 2 1/2 teaspoons kosher salt, divided; 2 teaspoons baking powder; 1 teaspoon five-spice powder; 1/2 teaspoon black pepper; 3/4 cup vegetable oil, divided; 2 teaspoons white sesame seeds , divided; Sour cream, for serving