Unfold phyllo, and cut into 2 (13- x 9-inch) rectangles; cover with damp cloth towels to prevent drying out.
Place 1 phyllo sheet in a buttered 13- x 9-inch baking dish; brush with melted butter. Place a phyllo sheet on top, allowing half of sheet to drape over 1 side of dish; brush portion in dish with melted butter. Repeat procedure with 15 more phyllo sheets, alternating sides of dish after each sheet so that each side of dish is covered.
Spread spinach mixture evenly in dish; top with a phyllo sheet, and brush with melted butter. Fold over 1 top and bottom overhanging phyllo sheet; brush with butter. Fold 1 overhanging sheet on each side toward center; brush with butter. Top with a phyllo sheet; brush with butter. Repeat folding procedure with the remaining overhanging phyllo sheets, topping each layer with a phyllo sheet and brushing with butter. Cut into 4 or 8 squares.
Bake at 425° for 20 minutes or until golden brown. Garnish, if desired.