Preheat your oven to 350°F. While it heats, prepare all your ingredients: finely dice the onion, chop the scallions (keeping white and light green parts separate from darker greens), roughly chop the spinach (if using fresh, or thaw frozen spinach and squeeze out excess moisture), crumble the feta into bite-sized pieces, chop the roasted red pepper into 1/2-inch pieces, and shred the mozzarella. Melt the butter and keep it warm for brushing. Having everything ready before you start cooking ensures you can work smoothly through the filling and assembly stages.

Heat the olive oil in a large skillet over medium-high heat. Add the diced onion and cook for 8 minutes, stirring occasionally, until softened and beginning to turn golden. Add the white and light green parts of the scallions and cook for 2 minutes. Now add the spinach, salt, and black pepper, stirring frequently until the spinach is completely wilted and any excess moisture has evaporated—this should take about 5-7 minutes depending on whether you started with fresh or frozen spinach. Remove from heat and let cool slightly.

Stir the crumbled feta and chopped roasted red pepper into the cooled spinach mixture from Step 2. Set the filling aside. Lightly brush a 9x13 inch baking dish (or similar-sized casserole) with melted butter, then sprinkle the breadcrumbs evenly across the bottom—the breadcrumbs act as a moisture barrier to keep the phyllo crust from getting soggy. I like to use breadcrumbs here rather than skipping this step because they truly make a difference in maintaining a crispy phyllo base throughout baking.

Working gently to avoid tearing, begin layering the phyllo sheets into the prepared baking dish. Brush each sheet lightly but thoroughly with melted butter before adding the next layer. You'll want to use about half the phyllo package (roughly 8-10 sheets) to create the bottom crust—each brushed sheet adds structural support and creates that delicate, flaky texture. The butter between layers is what transforms phyllo into something golden and crispy, so don't skip this step or be too timid with the brushing.

In a bowl, whisk together the eggs, half and half, ground nutmeg, salt, and black pepper until well combined and smooth. Pour this custard mixture into the phyllo-lined pan, spreading it evenly. Then distribute the spinach and feta filling from Step 3 over the custard layer, and sprinkle the shredded mozzarella on top. The custard will partially cook the filling and keep everything moist, while the mozzarella will melt and bind everything together.

Fold the edges of the phyllo sheets from the bottom layer up and over the filling to partially cover the top, creating a rustic, loosely enclosed appearance. Brush this folded top generously with the remaining melted butter. Layer the remaining phyllo sheets on top (about 8-10 sheets), brushing each one with butter just as you did with the bottom layers, then brush the final top layer generously with butter for a beautiful golden finish. Bake for 45 minutes, or until the phyllo is deep golden brown and a knife inserted into the center comes out hot, indicating the custard is set.

Remove the quiche from the oven and let it cool for at least 20 minutes before cutting and serving. This resting period allows the custard to fully set and the phyllo to crisp up further, making it much easier to slice cleanly without the filling running out. The quiche can be served warm or at room temperature, making it perfect for meals throughout the day.