From Cook's Illustrated
Microwaving, chopping, and then ridding mature fresh spinach of excess water gave our spanakopita recipe bold spinach flavor. To buffer the assertiveness of feta and to add textural contrast, we added Greek yogurt to the spanakopita recipe. We chose to bake our spanakopita on a baking sheet. Its sides were short enough to allow moisture to escape, crisping the phyllo dough’s bottom crust.
Full-fat sour cream can be substituted for whole-milk Greek yogurt. Phyllo dough is also available in large 14 by 18-inch sheets; if using, cut them in half to make 14 by 9-inch sheets. Don't thaw the phyllo in the microwave—let it sit in the refrigerator overnight or on the countertop for four to five hours. To make ahead, freeze the spanakopita on the baking sheet, wrapped well in plastic wrap, or cut the spanakopita in half crosswise and freeze smaller sections on a plate. Bake the spanakopita frozen, increasing the baking time by 5 to 10 minutes.
2 (10-ounce) bags curly leaf spinach, rinsed
1/4 cup water
12 ounces feta cheese, rinsed, patted dry, and crumbled into fine pieces (about 3 cups)
3/4 cup whole-milk Greek yogurt (see note)
4 medium scallions, sliced thin (about 1/2 cup)
2 large eggs, beaten
1/4 cup minced fresh mint leaves
2 tablespoons minced fresh dill leaves
3 medium garlic cloves, minced or pressed through garlic press (about 1 tablespoon)
1 teaspoon grated zest plus 1 tablespoon juice from 1 lemon
1 teaspoon ground nutmeg
1/2 teaspoon ground black pepper
1/4 teaspoon table salt
1/8 teaspoon cayenne pepper
7 tablespoons unsalted butter, melted
1/2 pound (14 by 9-inch) phyllo, thawed (see note)
1 1/2 ounces Pecorino Romano, grated fine (about 3/4 cup)
2 teaspoons sesame seeds (optional)
1. FOR THE FILLING: Place spinach and water in large microwave-safe bowl. Cover bowl with large dinner plate. Microwave on high power until spinach is wilted and decreased in volume by half, about 5 minutes. Using potholders, remove bowl from microwave and keep covered, 1 minute. Carefully remove plate and transfer spinach to colander set in sink. Using back of rubber spatula, gently press spinach against colander to release excess liquid. Transfer spinach to cutting board and roughly chop. Transfer spinach to clean kitchen towel and squeeze to remove excess water. Place drained spinach in large bowl. Add remaining filling ingredients and mix until thoroughly combined. (Filling can be made up to 24 hours in advance and stored in the refrigerator).
2. FOR THE PHYLLO LAYERS: Adjust oven rack to lower-middle position and heat oven to 425 degrees. Line rimmed baking sheet with parchment paper. Using pastry brush, lightly brush 14 by 9-inch rectangle in center of parchment with melted butter to cover area same size as phyllo. Lay 1 phyllo sheet on buttered parchment, and brush thoroughly with melted butter. Repeat with 9 more phyllo sheets, brushing each with butter (you should have total of 10 layers of phyllo).
3. Spread spinach mixture evenly over phyllo, leaving ¼-inch border on all sides. Cover spinach with 6 more phyllo sheets, brushing each with butter and sprinkling each with about 2 tablespoons Pecorino cheese. Lay 2 more phyllo sheets on top, brushing each with butter (these layers should not be sprinkled with Pecorino).
4. Working from center outward, use palms of your hands to compress layers and press out any air pockets. Using sharp knife, score pie through the top 3 layers of phyllo into 24 equal pieces. Sprinkle with sesame seeds (if using). Bake until phyllo is golden and crisp, 20 to 25 minutes. Cool on baking sheet 10 minutes or up to 2 hours. Slide spanakopita, still on parchment, to cutting board. Cut into squares and serve.