Slice the tops off the tomatoes and set aside. Scoop out the seeds and pulp from the tomatoes with a teaspoon, but be careful not to cut through to the base. Chop the pulp and keep the seeds, but discard the hard, central cores.
Heat the oil in a medium saucepan and tip in the rice. Fry gently over a low heat for 2 minutes, stirring frequently to stop the rice from browning, then add the scallions, zucchini and mushrooms and fry gently for 2 more minutes.
Preheat the oven to 375°F Pour in the stock then stir in the dried herbs, tomato pulp and seeds - the seeds will add fibre, and won't be noticeable in the finished dish.
Cook gently for about 15-20 minutes, stirring frequently until the rice is tender and the stock has been absorbed - add extra stock or water if necessary.
Turn off the heat, tear the basil leaves into the pan and scatter with parmesan. Season with salt and pepper and stir well.
Put the tomatoes in an ovenproof dish and fill each one generously with the risotto.
Replace tomato tops and bake, uncovered, for 15-20 minutes until the tomatoes are tender. Cool for 5 minutes, then serve with a few arugula leaves on top.