In a large mixing bowl, combine the flour, instant yeast, sugar, and salt. Mix well.
Gradually add the warm water and olive oil to the dry ingredients. Stir until a dough forms.
Knead the dough on a floured surface for about 5-7 minutes, until smooth and elastic.
Place the dough in a lightly greased bowl, cover with a damp cloth, and let it rise in a warm place for about 30 minutes, or until doubled in size.
Preheat your oven to 425°F.
Once the dough has risen, punch it down and divide it into two equal pieces. Roll each piece into a circle about 8 inches in diameter.
On one half of each circle, layer half of the mozzarella cheese, followed by half of the pepperoni. If using, sprinkle Italian seasoning on top.
Fold the other half of the dough over the filling and press the edges to seal. Use a fork to crimp the edges for a decorative touch.
Beat the egg in a small bowl and brush it over the top of each calzone for a golden finish.
Place the calzones on a baking sheet lined with parchment paper and bake for 15-20 minutes, or until golden brown.
Serve hot with marinara sauce for dipping.