How to Make It
Make the piecrust In a large bowl, whisk the 1 3/4 cups of flour with the salt. Using a pastry blender or your fingers, cut the butter and cream cheese into the flour until pea-size. Add the ice water and gently knead just until the dough comes together. Press into a disk, wrap in plastic and refrigerate until firm, at least 30 minutes.
On a lightly floured surface, roll out the dough to a 14-inch round. Ease into a 10-inch cast-iron skillet and trim the overhang to 1/2 inch. Fold the edge of the dough in over itself and, if desired, crimp it. Wrap in plastic and refrigerate for at least 30 minutes.
Preheat the oven to 350°. Line the piecrust with parchment paper and fill with pie weights or dried beans. Blind bake for about 1 hour, until the crust is set and lightly browned. Remove the weights and parchment paper, transfer the skillet to a wire rack and let cool completely. Leave the oven on.
Make the filling On a rimmed baking sheet, toss the scallions with the olive oil and thyme. Season with salt and pepper. Roast for 10 minutes, until tender. Let cool. Reserve 3 whole scallions. Coarsely chop the remaining scallions.
Reduce the oven temperature to 325°. In a large bowl, whisk the eggs with the cream, milk and 2 teaspoons of salt. Stir in the chopped scallions, the spinach, Gruyère and ricotta. Pour the custard into the piecrust and arrange the reserved whole scallions on top. Bake for 45 to 55 minutes, rotating halfway through, until the filling is puffed and lightly browned. Transfer to a rack and let cool for at least 20 minutes. Serve the pie warm or at room temperature.
In summer, sub in young stinging nettles (before they flower) for the spinach. Remember to wear gloves when handling raw nettles.