its literally just a pizza base folded over the filling do roll it out into a circle. Put all your fiolling in such as cheese, mushrooms, ham, peppers...whatever. Fold over and seal edges and then bake...Voila uno calzone.
Lying in the bath, thinking of calzone, as you do...and I remembered how I did it (I didn't have a recipe so just made it up based on ones I had in Italian restaurants).
Sauteed the onions and garlic in olive oil, added tomatoes and reduced it a bit so it was nice and intensely tomato-ey. Then stick your cooked chicken in and make sure it's warmed through. I'd cook the peppers in there a bit too so they're not too crunchy. And I'd chuck in some black olives, and lots of fresh basil leaves.
The other veg you don't need to cook, really - like mushrooms. Maybe wave the asparagus at a pan of hot water.
Then put the cooked filling on your pizza dough circles, put on some thick old chunks of mozzarella, fold over and do the Cornish pasty sylee crimping - and Bob's your uncle. If the fillings cooked already you don't need to worry about timings so much - just keep an eye on the crust and take it out when it's golden brown.