Heat a medium skillet over medium-high heat. Add butter and allow it to melt. Add the mushrooms and sauté for 4−5 minutes or until tender and lightly browned. Remove from heat and allow to cool.
Combine the green onion, garlic, spinach, blue cheese, chicken and reserved mushrooms in a bowl until blended. Season with salt and pepper.
Unroll pizza dough and place on a cookie sheet. Starting at the center, press out the dough into a 14-inch x 10-inch rectangle. Cut it into two rectangles. Spoon ½ of the calzone filling onto half of each rectangle, spreading to within a ½ inch of the edge. Fold dough in half over the filling lengthwise. Press edges firmly with a fork to seal. Prick tops with a fork. Bake for 20−25 minutes or until golden brown. Cut each calzone in half and serve immediately.