Heat oven to 375°F (190°C). In your largest bowl* place spinach, onion, scallions, garlic, herbs, and feta. Use your (freshly-washed, of course) hands to mix everything together, truly squeezing the feta into the other ingredients and breaking up the spinach a bit more as you do. I promise it’s fun. When the ingredients are tightly mixed, taste a pinch and add salt and pepper as needed to season it well — I use 1.5 teaspoons kosher salt and many grinds of black pepper. Add egg and mix until it disappears into the spinach.

Unwrap and unroll your phyllo so it’s in a flat pile and ready to use. I do not keep it covered with a cloth because we will use it fast.

Coat a 12-inch cast-iron skillet** or equivalent pan (12-inch cake pan or a 9×13-inch baking dish) generously with olive oil. You are going to want to be generous with the olive oil throughout this dish for the best flavor and texture; I estimate I use ½ cup total in this dish, but probably more.

Arrange 4 to 6 sheets [see Note about phyllo types] of phyllo around the pan, draping each across the bottom and letting the extra hang off over the side of the pan. Drizzle with olive oil. Rumple 2 to 3 sheets phyllo (one at a time) so they just cover the bottom of the pan. Drizzle with olive oil. Arrange half of the spinach mixture across the bottom. Scrunch 4 to 6 sheets of phyllo (think: hamburger-shaped) and arrange over spinach; drizzle with olive oil. Spoon remaining spinach mixture evenly over these scrunched sheets. Fold the parts of the phyllo sheets draped over the sides of the pan over the spinach filling, one at a time. Drizzle this closed top with more olive oil. One at a time, rumple remaining sheets so they fit over the top of the pan. Every layer or two, drizzle with more olive oil, and finish with a final drizzle olive oil.

Use a sharp, serrated knife ***, cut the spanakopita into serving-sized squares. Bake in preheated oven for 45 to 60 minutes, until the top is very crispy and nicely browned. Let cool on a rack for 15 minutes before serving; you’ll need to cut again, most likely, but it won’t mess up the pastry very much.

Leftovers keep for 1 week in the fridge (and could also be frozen). To reheat from the fridge, place uncovered in a 350-degree oven until warmed through and the pastry is crisp again, about 15 to 20 minutes.