Thaw spinach completely, then wrap in a clean kitchen towel and squeeze until no more liquid comes out. You should have about 1 cup firmly packed spinach. This step is crucial - excess water will make soggy pinwheels.

In a mixing bowl, beat cream cheese until smooth. Add sour cream, Parmesan, garlic, salt, pepper, and red pepper flakes. Fold in the squeezed spinach until evenly distributed. Taste and adjust seasoning.

Unfold one cold puff pastry sheet on a lightly floured surface. Roll gently to smooth creases, forming a 10x10 inch square. Spread half the spinach mixture in an even layer, leaving a 1-inch border along one long edge. Brush that bare edge with egg wash.

Starting from the filled long edge, roll tightly toward the egg-washed edge. Press gently to seal. Wrap in plastic and refrigerate while you repeat with the second sheet. Chill both logs for 15 minutes - this firms them up for clean slicing.

Preheat oven to 400°F. Line a baking sheet with parchment. Using a sharp knife, slice each log into 12 even rounds, about 3/4 inch thick. Place cut-side up 2 inches apart on the sheet. Brush tops lightly with remaining egg wash.

Bake 22-25 minutes until pastry is deep golden and puffed. Let cool 5 minutes on the sheet - they will crisp as they cool. Serve warm or at room temperature.