Heat oven to 375°F (190°C). In a large bowl, place spinach, onion, scallions, garlic, herbs, and feta. Mix everything together using your hands.
Add salt and pepper to taste, then mix in the egg until fully incorporated.
Coat a 12-inch cast-iron skillet or equivalent pan generously with olive oil.
Arrange 4 to 6 sheets of phyllo around the pan, drizzling with olive oil between layers. Scrunch 2 to 3 sheets and arrange over spinach; drizzle with olive oil.
Spoon half of the spinach mixture over the scrunched phyllo, then repeat with more scrunched phyllo and the remaining spinach mixture.
Fold the overhanging phyllo sheets over the filling and drizzle with olive oil.
Rumple remaining sheets over the top, drizzling with olive oil between layers.
Cut the spanakopita into serving-sized squares and bake for 45 to 60 minutes until crispy and browned.
Let cool for 15 minutes before serving.
