About 1 hour before baking, heat the oven to 500°F with a baking steel or stone on the upper-middle rack. Put the spinach in a large microwave-safe bowl. Cover and microwave on high until wilted, 2 to 4 minutes. Uncover and let cool slightly, then squeeze the spinach to remove excess moisture; wipe out the bowl. Roughly chop the spinach, then return it to the bowl and add the shallot, butter, garlic, lemon zest, paprika, pepper flakes (if using), 1 teaspoon salt and ½ teaspoon black pepper. Mix with your hands until well combined.

Lightly dust a baking peel, inverted baking sheet or rimless cookie sheet with semolina. Transfer one portion of the shaped dough to the peel and, if needed, reshape into a 6-by-12-inch oval. Spread half of the spinach mixture evenly over the dough, then top with half the feta. Slide the dough onto the baking steel and bake until the edges are golden brown, 9 to 11 minutes.

Using the peel transfer the flatbread to a wire rack. Top and bake the second portion of dough in the same way, then transfer to the wire rack.