Prepare the filling by placing a fine mesh strainer over a medium bowl; set aside.
Heat a large frying pan over medium-high heat until a drop of water sizzles in the pan, about 2 minutes. Add about half of the spinach and, using tongs, stir until it begins to wilt, then add the remaining spinach and cook, stirring occasionally, until completely wilted and bright green, about 5 minutes total. Transfer to the strainer.
Wipe out the frying pan with paper towels, return it to medium heat, and melt 2 tablespoons of the butter. When the foaming subsides, add the scallions and yellow onion, season lightly with salt, and sauté until softened, about 7 minutes. Transfer to a large bowl and set aside.
When the spinach is cool enough to handle, use both hands to squeeze out as much liquid as possible. (Excess liquid will make the pies soggy.) Stir the spinach into the reserved onions and cool slightly, about 3 minutes. Add feta, eggs, lemon zest, thinly cut mozzarella cheese and nutmeg. Season with freshly ground black pepper and stir to combine.
To assemble the pies, heat the oven to 375°F and arrange a rack in the middle. Line a baking sheet with parchment paper; set aside. Melt the remaining 14 tablespoons butter and place in a small bowl; set aside.
Dampen a tea towel or several pieces of paper towel with water. (Do not soak the towels or the phyllo will become soggy.) Clear a large work surface and lay the stack of phyllo with the short end closest to you.
Working quickly so as not to dry out the phyllo, gently slide one sheet onto the surface next to the stack. Cover the stack with the damp towel. Using a pastry brush, quickly and gently brush the entire surface of the single phyllo sheet with melted butter. Immediately remove the damp towel from the stack, lift a second phyllo sheet, and place it directly on top of the buttered sheet. Replace the damp towel and brush the second sheet with butter. Repeat until you have 4 layers of phyllo, brushing the top layer with butter.
Using a sharp knife, slice the buttered phyllo layers lengthwise into 3 equal pieces, each about 4 inches wide. Lay one of the phyllo rectangles perpendicular to you (with the narrow side closest to you). Place 2 rounded tablespoons of the filling 1 inch from the bottom edge. Fold the bottom right corner over the filling to the left edge to form a triangle. Fold the triangle up, so that the left bottom corner touches the left edge. Fold the lower left corner up and over to touch the right edge. Continue folding, flipping the triangle over and up (as if folding a flag), to the top. Trim any excess dough to form a perfect triangle. Place the triangle flat on the prepared baking sheet, seam side down. Repeat with the remaining phyllo pieces and filling.
Brush the tops of the triangles with butter and bake until puffed, flaky, and golden brown, about 30 minutes. Serve warm.
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