In a bowl, pour ½ cup of water and add yeast, mix and leave for 5 minutes until it appears foamy on top.
Add sugar, olive oil, remaining water, and salt to another bowl, and mix thoroughly. Once the yeast mixture is ready, add it to the flour mixture and combine.
Place the dough onto a floured work surface and knead for 3 to 5 minutes until you get a smooth, consistent, and elastic dough. Transfer dough to a bowl, cover with plastic wrap, and leave for 2 hours to rise at room temperature.
For the Filling
Pour oil into a skillet over medium heat, add onion and cook for 3 to 5 minutes until tender.
Add spinach and cook until it wilts completely.
Stir in the cooked chicken, Italian seasoning, garlic powder, and smoked paprika. Remove the skillet from heat.
Assembly
Once the dough is ready, divide into four equal portions and shape into balls. Roll them into a ¼" circular sheet using a rolling pin.
Spread marina sauce on each sheet, add chicken filling in the center, sprinkle mozzarella cheese on top, and fold the dough in half, covering all the filling.
Secure calzoni edges by pinching with fingers and brush the surface with egg. Arrange each calzoni on a baking sheet lined with parchment paper.
Place the baking sheet in a preheated (425 degrees F) oven and bake for 10 to 15 minutes until golden brown.