First, make your pizza dough. Preheat the oven to full whack, then tear the knocked-back dough into four pieces and roll each one out on a floured surface. You want to get them roughly circular, about the thickness of a pound coin, and 30cm across. You can now either keep these in the fridge, stacked and separated with olive-oil-rubbed and flour-dusted tinfoil, until you’re ready to cook them, or you can put your topping on and cook them straight away.
Pour a large lug 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 quickest 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 four 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 towards 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 tray (use two if you need to), 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.