To make the pizza dough, put the flour in a large bowl and stir in the yeast, 1 teaspoon olive oil and about 100ml warm water. Mix together until you have a soft smooth dough. Tip the dough out onto a lightly floured work surface and knead for a good 10-15 minutes until you have a soft stretchy dough.
Wash the spinach and place in a large saucepan with just the water that clings to its leaves. Heat gently until it wilts, then tip into a colander and leave to cool. When cool enough to handle, squeeze out the excess liquid. Heat the remaining oil and gently cook the finely chopped garlic for just a few seconds, then add the spinach and stir for just 1 minute.
Lightly dust the work surface with flour, then roll out the dough to 3mm thick, and cut out 4 x 10cm rounds. Spoon the spinach on one side of the circle. Make a well in the spinach, then crack an egg on top (crack it into a bowl first if you like). Sprinkle over the mozzarella. Fold over the dough and crimp to seal. Brush with milk and leave to rise in a warm place, covered with oiled cling film, for up to 40 minutes.
Meanwhile, preheat the oven to 220°C, fan 200°C, gas 7. Bake the calzones for 12-15 minutes until the pastry is puffed up and golden. The egg will be set inside.