Preheat the oven to 180°C (350°F)
Wash and dry the tomatoes then cut off the tops with a large knife. Do this carefully, not to damage the tops as they'll act as a lid when baking.
Using the side of a metal spoon cut the edges that connect the middle core to the outside skin of the tomato then scoop out the flesh, juice and seeds into a bowl. Once done, cover the bowl with cling film and store in a fridge, find tips for using the leftover juice below.
Sprinkle the hollowed out tomatoes with salt and sit them top side down on a plate to drain.
Meanwhile make the orzo filling. Finely chop the onion and add to a pan with a little olive oil, saute until soft and translucent. Add the sliced mushrooms and garlic and cook down for around 3 minutes until soft and reduced in size.
Add the orzo pasta and stir to coat in the mushroom and onion mixture. Next add ¼ of the chicken stock and stir as if you're making a risotto until the liquid has absorbed. Continue to add the stock ¼ at a time until you've used it all up
Taste and add a good pinch of salt if needed, if the pasta is still under cooked then add more stock until soft. Once done add the parmesan and stir until melted and creamy
To assemble the tomatoes, fill each tomato half way with orzo then top with a few cubs or slices of mozzarella. Add another layer of orzo (they can be slightly mounted) then top with the remaining mozzarella. Place the tomato lids on top of each tomato then bake in the oven for 30 minutes