Thaw the 2 packages frozen chopped spinach completely, then transfer it in batches to a clean kitchen shears towel or several layers of cheesecloth. Wring it out as firmly as you can — you want it as dry as possible. This is the single most important step: excess moisture will make your pie soggy. Set aside.

Heat the 3 tbsp olive oil in a skillet over medium heat. Add the diced yellow onion finely diced and cook, stirring occasionally, until softened and translucent, about 5–6 minutes. Add the 4 cloves garlic minced and cook for 1 more minute until fragrant. Remove from heat and let cool slightly.

In a large mixing bowl, combine the wrung-out 2 packages frozen chopped spinach, sautéed 1 yellow onion finely diced and 4 cloves garlic minced, beaten eggs lightly, crumbled 16 oz feta cheese crumbled, 1 cup ricotta cheese, ½ cup fresh dill roughly chopped, ¼ cup fresh flat-leaf parsley roughly chopped, 4 stalks green onions thinly sliced, ½ tsp ground nutmeg, and ½ tsp black pepper freshly ground. Mix thoroughly until everything is evenly combined. Taste the mixture and add ¼ tsp kosher salt taste before adding only if needed — 16 oz feta cheese crumbled is quite salty on its own. Set the filling aside.

Preheat your oven to 375°F. Brush a 9x13-inch baking dish generously with melted unsalted butter. Unroll the 1 package phyllo dough thawed overnight in the refrigerator and lay it flat. Immediately cover it with a sheet of plastic wrap and then a slightly damp kitchen shears towel — 1 package phyllo dough thawed overnight in the refrigerator dries out and cracks within minutes when exposed to air, so keep it covered whenever you're not actively working with it.

Lay one sheet of phyllo dough into the buttered baking dish, letting any excess drape over the sides. Brush it all over with melted unsalted butter. Repeat with 7 more sheets (8 total), brushing each one with ¾ cup unsalted butter melted (about 1.5 sticks) before adding the next. This forms your bottom crust.

Spread the 2 packages frozen chopped spinach and 16 oz feta cheese crumbled filling evenly over the 1 package phyllo dough thawed overnight in the refrigerator base, smoothing it into the corners with a spatula.

Layer 8 more sheets of phyllo dough on top of the filling, again brushing each sheet generously with ¾ cup unsalted butter melted (about 1.5 sticks). For the final top sheet, tuck any overhanging edges down and inward along the sides of the dish to create a neat border, and brush the top thoroughly with ¾ cup unsalted butter melted (about 1.5 sticks).

Using a very sharp knife, score the top layers of phyllo dough (don't cut all the way through) into the portion sizes you want — typically 8 to 12 squares or rectangles. This prevents the top from shattering uncontrollably when you cut it after baking. Sprinkle a few drops of water on top to help prevent the edges from curling.

Bake in the preheated oven for 45–50 minutes, until the top is deep golden brown and the 1 package phyllo dough thawed overnight in the refrigerator is crisp and flaky. If the top is browning too fast, tent loosely with aluminum foil after 30 minutes.

Remove from the oven and let the spanakopita rest for at least 10–15 minutes before cutting. This allows the filling to set so slices hold their shape. Cut along your scored lines and serve warm or at room temperature.