01 - Combine flour and salt in a large bowl. Add cold butter cubes and rub into flour until mixture resembles coarse breadcrumbs. Whisk egg and incorporate into mixture, adding cold water 1 tablespoon at a time until dough comes together. Form into a disk, wrap in plastic, and refrigerate for 30 minutes.
02 - Set oven to 375°F. Generously butter a 9-inch tart pan with removable bottom.
03 - Roll chilled dough on a lightly floured surface to 1/8-inch thickness. Carefully transfer to prepared pan, pressing dough into corners and trimming excess dough from edges.
04 - Whisk ricotta, chopped spinach, Parmesan, 3 eggs, heavy cream, nutmeg, salt, and pepper in a large bowl until completely 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 shallow wells in the filling.
06 - Carefully crack 1 medium egg into each well, taking care not to break yolks. Gently spoon remaining ricotta mixture around and over eggs, leaving yolks exposed.
07 - Bake for 40-45 minutes until filling is completely set and top is golden brown. Center should not jiggle when gently shaken.
08 - Let tart cool in pan for at least 15 minutes before removing. Serve warm or at room temperature.