How to Make It
In a large bowl, whisk the flour with 1 teaspoon of salt. Working over the bowl, grate the frozen butter on the large holes of a box grater. Gently toss the grated butter in the flour. Stir in 1/3 cup of ice water until the dough is evenly moistened; add more water if the dough is too dry. Scrape out onto a work surface, gather up any crumbs and knead gently just until the dough comes together. Cut the dough in half and pat them into squares. Wrap the squares in plastic and refrigerate until chilled, about 1 hour.
Meanwhile, in a large saucepan, heat the olive oil until shimmering. Add the onion, garlic, crushed red pepper, and a generous pinch each of salt and pepper. Cook over moderate heat, stirring occasionally, until the onion is softened, about 8 minutes. Add the collard greens and spinach in large handfuls, letting each handful wilt slightly before adding more, then add the wine. Cover and cook, stirring occasionally, until the mixed greens are wilted and just tender, about 10 minutes. Remove from the heat and let cool. Using a potato ricer, squeeze the excess liquid from the mixed greens and transfer them to a large bowl. Stir in the sour cream and nutmeg; season with salt and pepper.
Preheat the oven to 400°. On a floured work surface, roll out 1 piece of the dough to a 13-by-9-inch rectangle. Slide the dough onto a large baking sheet lined with parchment paper. Repeat with the second piece of dough. Refrigerate for 15 minutes.
Slide 1 rectangle of dough onto a work surface. Scatter the mixed greens evenly on the dough rectangle on the baking sheet, leaving a 1-inch border. Ease the other rectangle of dough on top of the mixed greens. Fold over the edge and crimp decoratively all around to seal. Brush the pie with the egg wash and sprinkle with flaky sea salt and freshly ground black pepper. Cut 24 small slits in the top and freeze for 15 minutes.
Bake the pie for about 50 minutes, until golden and crisp; rotate halfway through baking. Let cool for 15 minutes. Cut the pie into squares and serve.