01 - Combine flour and salt in a large bowl. Rub cold butter into flour until mixture resembles coarse breadcrumbs. Add egg and 2-3 tablespoons cold water, mixing until dough forms a ball. 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 - Roll chilled dough on a floured surface to 1/8-inch thickness. Carefully transfer to tart pan, pressing dough into corners. Trim excess dough from edges.
04 - Whisk together ricotta, chopped spinach, Parmesan, 3 eggs, heavy cream, nutmeg, salt, and pepper until smooth and well combined.
05 - Spread half the ricotta mixture into tart shell. Create 4 evenly spaced wells using the back of a spoon. Crack a medium egg into each well, taking care not to break yolks.
06 - Gently spoon remaining ricotta mixture around and over the eggs, covering whites while leaving yolks partially exposed.
07 - Bake for 40-45 minutes until filling is completely set and top is golden brown. Eggs should be fully cooked with firm whites.
08 - Let tart cool in pan for at least 15 minutes before removing. Slice and serve warm or at room temperature.