Heat oven to 350°F. Place 1 pie crust in 9-inch glass pie plate as directed on box for One-Crust Filled Pie. In 12-inch skillet, melt butter over medium heat. Cook onion in butter 5 minutes, stirring occasionally, until tender. Stir in flour, salt and pepper. Cook and stir 1 minute. Stir in spinach; cook 1 minute longer.
In large bowl, mix cheeses and 3 of the eggs. Add spinach mixture; stir until well blended. Spoon filling into crust-lined pie plate. Unroll second crust on work surface. Using ghost-shaped cookie cutter or other Halloween cookie cutter, cut out 3 shapes. Arrange cutouts on spinach mixture. Beat remaining egg; brush over dough cutouts.
Bake 45 minutes or until set and top is golden brown. Cool 10 minutes before cutting.