2) In a large mixing bowl, whisk together flour and salt.
3) With a rubber spatula, gradually mix the yeast mixture into the flour until just combined and dough barely holds together. Turn the dough out onto a clean counter surface dusted with flour.
4) Knead the dough with flour-dusted hands until the dough has become smooth and elastic, about 10 minutes. The surface should be tight and silky and bounce back slightly when pressed. Lightly grease a large mixing bowl with olive oil and place the dough in it.
5) Cover the bowl tightly with clingfilm and set in a warm place to rise until doubled in size, 1-2 hours.
6) Once doubled, punch the dough down and divide it into two equal pieces. Knead each one slightly to form a uniform ball. Set aside and cover them with a kitchen towel or clingfilm to rest for 30 minutes.
7) Tightly wrap one of the dough balls in clingfilm or in a box with a tight-fitting lid and refrigerate for other uses.
8) For the calzone, preheat oven to 230C/Gas 9.
9) Lightly dust the work surface and rolling pin with flour. Roll the remaining dough ball into a 20-by-30cm oval.
10) In a medium bowl, mix the spinach, ricotta and mozzarella and season with salt and pepper to taste.
11) Place all the filling on one half of the dough leaving a 2cm border along the edge.
12) Fold the remaining dough over the filling until the edges line up and pinch the edges together to seal. Gently roll the pinched edges under to form a decorative rim.
13) Place the calzone on a baking sheet dusted with cornmeal and brush the top with olive oil.
14) Bake until golden brown and the centre is hot and melted, about 25 minutes, rotating halfway through cooking.
Cook's Note: Dough can be made the day before and refrigerated overnight once it is divided. Keep covered tightly in clingfilm or in a sealed food storage bag.