Meanwhile, prepare the filling. Heat the olive oil in a cast-iron skillet. Fry the onions and crushed garlic for 2-3 minutes, stirring frequently. Add the frozen spinach and a little water, then cover the skillet with a lid and let the spinach cook for 5 minutes, stirring frequently. When all the liquid has evaporated, the spinach is ready. Remove from the heat, transfer to a bowl and allow to cool.
Once the spinach has cooled, add the crumbled goat cheese, season with salt and pepper and stir well to combine.
Preheat the oven to 200°C/400°F.
Divide the dough in 4 portions. Roll out each dough portion on a lightly floured surface.
Spread one tablespoon of pizza sauce on each dough circle.
Then place 1/4 of the filling mixture onto one half of each dough circle, leaving a 1inch gap around the edge. Carefully lift the far edge of the dough circle, pulling it over the top to cover the filling so that it is folded in half, resembling a half-moon shape. Seal the edges of dough by pressing it with a fork. Use a sharp knife to make small cuts on top of the calzone.
Brush calzone with egg yolk, to achieve a beautiful golden color.
Transfer calzones onto a preheated baking sheet lined with parchment paper, and bake for 7-10 minutes or until the dough is cooked through and the filling hot. Remove from oven, allow to cool and serve immediately, or pack for a picnic.