Preheat oven to 400°F.

In small microwave-safe bowl, heat spinach in microwave on High for 2 to 3 minutes, until spinach is mostly thawed but still cool enough to handle.

Squeeze spinach to remove excess water.

Meanwhile, in small bowl, combine ricotta, mozzarella, cornstarch, and oregano; set aside.

Coat large cookie sheet with nonstick cooking spray.

Unroll pizza dough on center of cookie sheet.

With fingertips, press dough into 14” by 10” rectangle.

Sprinkle cheese mixture lengthwise over half of dough, leaving 1-inch border.

Spoon marinara sauce over cheese mixture; top with spinach.

Fold other half of dough over filling.

Pinch edges together to seal.

Bake calzone until well browned on top, 20 to 25 minutes.

Cut into 4 equal pieces to serve.