Skillet Spanakopita

Name: Skillet Spanakopita
Ingredients: 1 1/2 lb. (750 g) frozen chopped spinach; 3 Tbs. extra-virgin olive oil; 1 small yellow onion, diced; 2 garlic cloves, minced; Kosher salt and freshly ground pepper; 2 whole eggs plus 1 egg yolk; 1/3 cup (1/2 oz./15 g) chopped fresh flat-leaf parsley; 4 green onions, white and light green portions, chopped; 2 Tbs. chopped fresh dill; 1 cup (5 oz./155 g) crumbled feta cheese (preferably sheep’s milk), plus more for garnish; 4 Tbs. (1/2 stick) (2 oz./60 g) unsalted butter, melted and cooled slightly; 15 sheets phyllo dough, thawed