First, make your Pizza Dough. Preheat the oven to 450 to 500 degrees F, then tear the punched-down dough into 4 pieces and roll each out on a floured surface. You want to get them roughly circular, about the thickness of a silver dollar, and 12 inches across. You can now either keep these in the refrigerator, stacked and separated with olive-oil-rubbed and flour-dusted aluminum foil, until you're ready to cook them, or you can put your topping on and cook them straightaway.
Pour a large glug of olive oil into a hot frying pan. Add the mushrooms and toss briefly in the hot oil before adding the sliced garlic and the thyme. Fry until the mushrooms are cooked and smell fantastic. Drop in the butter and toss the mushrooms in it to make them tasty and shiny. Season with a little salt and pepper.
Add the tomato sauce to the pan and stir. Cook for a few minutes, then add the spinach (in batches if you need to) and stir again. Simmer away the liquid until you're left with a thick, tasty mixture that's not too moist (otherwise it will burst through the dough when you're cooking the calzone).
Divide the mushroom and spinach mixture evenly between the 4 pizza bases and spread it out nicely. Top with pieces of mozzarella and season with salt and pepper. To make your calzone, carefully lift the far edge of the Pizza Dough and pull it over the top toward you - you basically need to fold it in half (imagine it looking like a big Cornish pasty!). Crimp the edges so none of the filling can spill out. Place the calzone side by side on a floured baking sheet, (use 2 if needed), pizza stone or granite slab.
Cook for 10 to 15 minutes on the bottom of the preheated oven until the dough is puffed up and golden on top and the filling is hot.
Sift the flours and salt onto a clean work surface and make a well in the middle. In a large measuring cup, mix the yeast, sugar and olive oil into the water and leave for a few minutes, then pour into the well. Using a fork, bring the flour in gradually from the sides and swirl it into the liquid. Keep mixing, drawing larger amounts of flour in, and when it all starts to come together, work the rest of the flour in with your clean, flour-dusted hands. Knead until you have a smooth, springy dough.
Place the ball of dough in a large flour-dusted bowl and flour the top of it. Cover the bowl with a damp cloth and place in a warm room for about 1 hour until the dough has doubled in size.
Now remove the dough to a flour-dusted surface and knead it around a bit to push the air out with your hands - this is called punching down the dough. You can either use it immediately, or keep it, wrapped in plastic wrap, in the fridge (or freezer) until required. If using straightaway, divide the dough up into as many little balls as you want to make pizzas - this amount of dough is enough to make about six to eight medium pizzas.
Timing-wise, it's a good idea to roll the pizzas out about 15 to 20 minutes before you want to cook them. Don't roll them out and leave them hanging around for a few hours, though - if you are working in advance like this it's better to leave your dough, covered with plastic wrap, in the refrigerator. However, if you want to get them rolled out so there's 1 less thing to do when your guests are round, simply roll the dough out into rough circles, about 1/4-inch thick, and place them on slightly larger pieces of olive-oil-rubbed and flour-dusted aluminum foil. You can then stack the pizzas, cover them with plastic wrap, and pop them into the refrigerator.
Recipe Excerpted from JAMIE AT HOME by Jamie Oliver. Copyright (c) 2008. Published in the U.S. by Hyperion. All Rights Reserved. Available wherever books are sold. www.jamieoliver.com