Preheat oven to 350 degrees. Heat oil in a large skillet over medium heat. Add onion, and cook until softened, about 8 minutes. Add garlic, and cook, stirring, 1 minute. Gradually add spinach, and cook, stirring often, until wilted, about 5 minutes. Transfer to a bowl. Stir in egg, herbs, feta, salt, and pepper.
Lay 1 sheet phyllo on a parchment-lined baking sheet (cover remaining phyllo with a damp towel), and brush with butter. Repeat twice. Spread spinach mixture on center. Top with 3 more buttered sheets. Fold in sides. Butter remaining 4 sheets; crumple over top. Bake until dark golden, about 40 minutes.