Make the filling: In a large nonstick skillet, heat 2 tablespoons oil over medium-high heat. Add onions, and cook, stirring occasionally, until translucent, 3 to 5 minutes. Add garlic and 2 teaspoons salt; cook until garlic is tender, 1 to 2 minutes.
Transfer mixture to a large bowl; stir in spinach, feta, Parmesan, breadcrumbs, dill, 2 teaspoons salt, and ½ teaspoon pepper. Fold in eggs until combined.
Divide mixture evenly between two 9-inch springform pans or two 9 ½-inch deep-dish pie plates; press firmly to flatten.
Make the topping: In a large bowl, gently toss sliced phyllo to separate, then toss with remaining ½ cup oil until coated.
Divide phyllo between pies, covering the tops evenly and completely. (To freeze, cover pies tightly with plastic wrap, being careful not to flatten topping. Bake within 3 months; do not thaw first.)
Preheat oven to 375°. Bake until heated through and topping is golden brown, about 1 hour 15 minutes for frozen pie (30 minutes for unfrozen pie).