Pour water in a mixing bowl, then add sugar and yeast. Thoroughly combine, and leave it for five minutes until foamy.
In another mixing bowl, add butter, salt, and flour. Mix well.
Pour the activated yeast mixture into the flour mixture and knead the dough for three to five minutes until the dough becomes smooth.
Add a little olive oil in the bowl, coat using a brush and add the formed dough to the bowl. Then cover the bowl using plastic wrap and leave it in a warm, dry place to rise for about an hour, or until doubled in size.
For the Filling
Pour olive oil into a skillet over medium-high heat. Add the garlic and onion, and sauté for five minutes until they soften.
Add half of the spinach and cook it down until it wilts.
Next, add the remaining spinach, bell peppers, Prosciutto, and artichoke to the pan.
Season with Italian seasoning, black pepper and salt. Mix well.
Cook until all the spinach wilts thoroughly and everything is warmed through.
Assembly
Once the dough is ready, lightly punch it down and divide into four equal portions. Take one portion and roll it into a circular ball.
Flatten the dough ball on a floured workstation, rolling it out with a rolling pin until it is about 1/8" thick. Repeat the same steps with the other portions.
Put filling in the center of each dough circle, fold it over, and secure the edges by pinching with your fingers. Brush egg yolk over the surface of calzonis and arrange them on a baking sheet lined with parchment paper.
Transfer the baking sheet to a preheated oven (425 degrees F) and bake for 16 to 20 minutes or until golden brown.
Once done, remove calzonis from the oven, and brush some olive oil on top. Sprinkle Italian seasoning on top and serve immediately.