This is a classic recipe from Rome and one which can be successfully replicated also here in the UK, where good tomatoes are rare. However, in order to coax as much flavour as possible from the often lustreless UK, tomatoes, I have employed few tricks- a long, slow roasting, some flavour enhancers and careful seasoning.
After comparing few sources (Carla Tomasi and Rachel Roddy and Frank Fariello and Domenica Marchetti), I went back to the book that is considered one of the grand books on Italian regional cooking: La Cucina Romana, by Ada Boni (1928). Most recipes for pomodori col riso are similar, Ada Boni only however mentions “cinnamon”: adding a pinch of cinnamon to each tomato, gives this humble dish an exotic, fuller taste, much appreciated.
This is what I did;
Slice off the tops of the tomatoes and scoop out as much pulp as possible and the seeds, leaving an empty shell that is about 2 cm thick.
Cut up any large-ish piece of pulp: by the end you should be left with a mix that is brothy and full of rice-grain size pieces of tomato pulp (what u see in the picture is the tomato pulp pieces still uncut). Add a couple of teaspoons of tomato paste, mix well and one and a half tablespoon risotto rice per tomato.
Dress the mix with salt, pepper, a pinch of sugar, a glug of oil, chopped parsley (maybe with some chopped mint leaves) and finely chopped garlic: as usual, authentic Italian cooking is discreet with garlic and I used only one fat clove for 5 large beefsteak tomatoes .
Carefully and thoroughly, sprinkle the inside of each tomatoe with salt, pepper, a pinch of sugar and a pinch of cinnamon. Dribble a little olive oil too (1 teaspoon per tomato).
Place the tomatoes in a large oven dish, already oiled and splashed with water. Fill the tomatoes, making sure to distribute the rice evenly (it helps if you give a good stir to the mix each time you fill a tomato) cover them with their lids, drizzle some more oil all over and bake at 170 C for a couple of hours. You want a long and slow bake to concentrate the tomato flavour and also to avoid splitting the tomato skin, which would happen at higher temperature. I also have a thing with not properly cooked baked/roasted tomatoes (I hate them): for me a cooked tomato, must be FULLY COOKED, meltingly tender but still retaining their shape for me. Half cooked tomatoes are disgusting. I pushed the baking to 2.5 hrs once and they were delicious. No, the rice does not overcook, because the temperature is gentle and because it is not swimming in liquid.
Let them rest and eat no warmer than room temperature. In Rome, they sometimes add potatoes, cut into wedges (dressed with oil and salt before going into the pan). If you do this, make sure not to crowd the dish, otherwise the whole lot will steam instead of roasting, so to speak.
A typical example of cucina degli avanzi, leftovers cooking. I had some leftover mussels from the night before (cozze alla marinara, mussels cooked in a tomato, parsley and garlic broth) and, on the spur of the moment, I decided to transform them into a rice and mussels dish, for supper. 
