In a skillet, cook the sausage and onion over medium heat for about 5 minutes,
stirring to break up the meat.
Add bell pepper and mushrooms and cook for 5 minutes
more, stirring often, or until the sausage is cooked through and vegetables are soft.
Remove with a slotted spoon to a large bowl.
Stir in the mozzarella, parsley and red pepper flakes.
Heat the oven to 450 degrees F. Brush 2 baking sheets with olive oil or spray
with olive oil cooking spray.
Divide the dough into 4 pieces. Take one piece of the dough and with floured
hands press it into a 6-inch round. Stretch or roll it out with a rolling pin into
a 12-inch circle. Leaving a 1/2-inch edge, spread a quarter of the filling over
half of the dough. Dampen the edges with water. Fold the dough in half to enclose
the filling; seal the edges with a fork. Repeat with the remaining dough and filling.
With a large spatula, carefully transfer the calzones to the prepared baking sheets.
Bake for 20 minutes, or until golden and crisp on the outside.