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 in plastic, and refrigerate for 30 minutes.
02 - Preheat oven to 375°F. Lightly grease a 9-inch tart pan with butter or cooking spray.
03 - On a lightly floured surface, roll out chilled dough to 1/8-inch thickness. Carefully transfer to prepared tart pan, pressing gently into bottom and up sides. Trim excess dough from edges.
04 - In a large bowl, whisk together ricotta, chopped spinach, Parmesan, 3 eggs, heavy cream, nutmeg, salt, and pepper until smooth and well combined.
05 - Spread half of ricotta mixture evenly into prepared tart shell. Using back of spoon, create 4 evenly spaced shallow wells in filling.
06 - Carefully crack one medium egg into each well, taking care not to break yolks.
07 - Gently spoon remaining ricotta mixture around and over eggs, covering whites while leaving yolks partially exposed.
08 - Bake for 40-45 minutes until filling is completely set and top is lightly golden brown. A knife inserted in center should come out clean.
09 - Let tart cool in pan for at least 15 minutes before removing. Slice and serve warm or at room temperature.