01 - Combine flour and salt in a large bowl. Cut in cold butter until mixture resembles coarse crumbs. Add egg and 2-3 tablespoons cold water, mixing just until dough holds together. Form 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 - Roll chilled dough on a floured surface to 1/8-inch thickness. Carefully transfer to prepared tart pan, pressing dough into sides and trimming excess from edges.
04 - Whisk together ricotta, chopped spinach, Parmesan, 3 eggs, cream, nutmeg, salt, and pepper until smooth and well combined.
05 - Spread half the ricotta mixture evenly over the bottom of the tart shell.
06 - Using the back of a spoon, make 4 evenly spaced indentations in the filling. Carefully crack a whole egg into each well without breaking yolks.
07 - Gently spoon remaining ricotta mixture around and over the eggs, covering the whites while leaving yolks exposed.
08 - Bake for 40-45 minutes until filling is completely set and top is golden brown. The center should not jiggle when gently shaken.
09 - Let tart rest for at least 15 minutes before removing from pan. Slice and serve warm or at room temperature.