Made from store-bought pizza dough, this savory pie filled with fresh spinach, mozzarella, and feta is a cinch to put together.
Place spinach in a large pot over high heat; cover. Steam until just collapsed, about 3 minutes; transfer to a colander. When cool enough to handle, squeeze out excess water; roughly chop.
Preheat oven to 425 degrees, with a rack in lower third. Brush a 9 1/2-by-13-inch rimmed baking sheet with oil. On a lightly floured work surface, roll out and stretch one piece of dough into a thin 10-by-13-inch rectangle. (If dough is springing back, let stand 5 minutes.) Transfer to prepared baking sheet. Arrange mozzarella over dough, leaving a 1-inch border. Top with spinach and sprinkle with feta. Season with salt and red-pepper flakes. Roll out and stretch remaining portion of dough into a 10-by-14-inch rectangle. Drape over filling. Pinch edges to seal; slash a few vents in top. Brush with egg wash; sprinkle with sesame seeds.
Bake until golden brown all over and filling is bubbling through vents, 40 to 45 minutes. (Check color of bottom crust by lifting corner with a spatula.) Immediately slide pie off sheet onto a wire rack. Let cool completely. Before serving, cut into squares.
Letting the pie cool on a wire rack keeps the bottom crust from getting soggy. Once it's cool, return it to the baking sheet for transport.