Preheat oven to 425 degrees F.

In a medium-size bowl, combine ricotta and Asiago cheeses, spinach, salt, and pepper to taste.

Set aside the filling.

On a lightly floured surface, unroll pizza crust.

Roll out the crust to a 14-inch square.

Using a 3 1/2-inch round cookie cutter, cut out 12 circles.

Brush the edges of each circle with some of the egg yolk mixture.

Place 1 teaspoon of filling on the lower half of each circle.

Fold the pastry over to enclose the filling, pressing the edges with a fork to seal.

Brush with the remaining egg yolk mixture.

Prick the top of each calzone with the point of a sharp knife.

Sprinkle with additional grated Asiago cheese.

Place the calzones on a large, greased cookie sheet.

Bake for 5 to 7 minutes, until puffed and golden.

Cool slightly before serving.