How to Make It
Preheat oven to 375°F. Heat oil in a 10-inch ovenproof skillet over medium-high. Add leek and garlic and cook until softened, about 3 minutes. Season with pepper and ¼ teaspoon salt. Transfer to a large bowl and add spinach, peas, feta, dill, lemon zest and juice, oregano, eggs, and remaining ¾ teaspoon salt.
Wipe skillet clean and brush with 1 tablespoon butter. Unroll phyllo (keep covered with a dry kitchen towel). Transfer 1 sheet to a clean surface. Brush with 1 tablespoon butter and place, buttered side up, in skillet, pressing into bottom; fold any overhang toward center. Repeat with 3 more sheets and 3 tablespoons butter.
Brush another phyllo sheet with 1 tablespoon butter. Place half of sheet, buttered side up, in skillet so half is hanging over rim. Repeat with 3 tablespoons butter and remaining 3 sheets, placing so dough hangs all around rim of skillet.
Spread spinach mixture in skillet. Fold overhang toward center and brush with remaining 1 tablespoon butter.
Bake until top is golden brown and crispy, 40 to 45 minutes. Let rest for 20 minutes before serving.