Preheat oven to 350 degrees. Oil a 9 × 13-inch baking dish.
Heat 2 tablespoons of the olive oil in a large pot over medium heat. Add the scallions and the onion, and sauté until translucent, about 5 to 6 minutes. Add the spinach and sauté until just wilted. Remove from heat, let cool slightly, and transfer to a fine-mesh strainer. Cool slightly, then squeeze as much excess water as possible from the spinach, and transfer to a large mixing bowl.
Crumble the feta into small pieces and add it to the spinach mixture. Add the dill and mix gently to combine. Add the eggs and pepper. Mix well to combine with impeccably clean hands or a silicone spatula.
Roll the phyllo dough out on a flat surface, working quickly and keeping it covered to prevent it from drying out. Place 2 phyllo sheets into the baking dish at a time, centering them and letting the edges hang over the sides. Brush the top sheet of each 2-sheet layer with a little of the remaining olive oil, but do not brush the overhanging edges. Continue in this manner until you have used 10 of the phyllo dough sheets.
Spread the spinach-feta mixture evenly over the phyllo dough layers in the prepared dish. Fold the overhanging phyllo dough over the filling, then continue to layer the phyllo dough, brushing each 2-sheet layer with olive oil, until you have used all of the dough. Trim the top layers of phyllo to fit the baking dish. Slowly pour the remaining olive oil on top, and spread evenly.
Before baking, using a large knife, very carefully score the pie into 8 to 10 pieces, cutting through the top layers just until you reach the filling. Precutting makes it much easier to serve, as the phyllo dough becomes crisp and very fragile after baking.
Bake for 50 to 60 minutes or until the top is golden brown and flaky, watching carefully. Cool for 10 minutes, slice the precut pieces all the way through, and serve.
Debbie’s Tip: As noted earlier, as with most of the phyllo pies, spanakopita can be made with either melted butter or extra-virgin olive oil. I find the olive oil version to be a lighter, more healthful dish, but I also love to indulge in my mom’s decadent, traditional version once in a while. To make it Mom’s way, replace the olive oil used to brush the phyllo with 6 tablespoons of melted butter (see tip, page 112), add an extra egg, and use 11/2 pounds of feta instead of 1 pound. It’s over-the-top delicious!