01 - Combine flour and salt in a large bowl. Add cold butter cubes and rub into flour until mixture resembles coarse breadcrumbs. Add egg and 2-3 tablespoons cold water, mixing until dough just comes together. Shape into a disk, wrap tightly, and refrigerate for 30 minutes.
02 - Preheat oven to 375°F. Generously grease a 9-inch tart pan with butter or cooking spray.
03 - On a lightly floured surface, roll out chilled dough to a 11-inch circle. Carefully transfer dough to prepared tart pan, pressing gently into sides and bottom. Trim excess dough from edges.
04 - In a large bowl, whisk together ricotta, chopped spinach, Parmesan, 3 eggs, heavy cream, nutmeg, salt, and black pepper until smooth and well combined.
05 - Spread half of the ricotta mixture evenly into the prepared tart shell. Using the back of a spoon, create 4 evenly spaced shallow wells in the filling.
06 - Carefully crack one medium egg into each well, taking care not to break the yolks.
07 - Gently spoon remaining ricotta mixture around and over the eggs, covering the whites while leaving yolks exposed. Ensure the filling evenly distributes without disturbing the egg yolks.
08 - Bake for 40-45 minutes until filling is completely set and top is lightly golden. The center should not jiggle when gently shaken.
09 - Let tart cool in pan for at least 15 minutes before removing. Slice and serve warm or at room temperature.