01 - Combine flour and salt in a large bowl. Rub cold butter into the flour mixture until it resembles coarse breadcrumbs. Add egg and 2-3 tablespoons cold water, mixing until dough just comes together. Form into a disk, wrap in plastic, and refrigerate for 30 minutes.
02 - Preheat oven to 375°F. Generously grease a 9-inch tart pan with butter or cooking spray.
03 - Roll out chilled dough on a floured surface to fit the tart pan. Carefully transfer dough to pan, pressing into the bottom and up the sides. Trim any excess dough from edges.
04 - Whisk together ricotta, chopped spinach, Parmesan, 3 eggs, heavy cream, nutmeg, salt, and black pepper in a large bowl until smooth and well combined.
05 - Spread half the ricotta mixture evenly across the bottom of the tart shell. Using the back of a spoon, create 4 evenly spaced indentations in the filling.
06 - Carefully crack one medium egg into each well, keeping yolks intact. Gently spoon remaining ricotta mixture around and over the eggs, covering the whites while leaving yolks exposed.
07 - Bake for 40-45 minutes until filling is completely set and top is lightly golden brown. The center should not jiggle when gently shaken.
08 - Let tart cool in pan for at least 15 minutes to allow filling to set completely. Slice into wedges and serve warm or at room temperature.