Cut the tops off the tomatoes and scoop out the seeds and pulp. Push the pulp and tomato liquid through a sieve into a bowl and reserve, discarding the seeds.
Season the insides of the tomato shells and leave upside down on a plate to drain.
Heat the oil in a pan and fry the onion until soft. Add the mince, increase the heat slightly and fry the mince until it starts to brown. Lower the heat again and add the mushrooms, red pesto, stock and reserved tomato pulp and liquid.
Stir well, cover the pan and simmer for 15 minutes. Remove the lid and simmer for a further 5 minutes so the liquid reduces and the mixture is quite thick. Leave to cool, then pack into the tomato shells and replace the lids.
Preheat the oven to 180C/350F/Gas 4. Stand the tomatoes in a shallow ovenproof dish and bake them uncovered for 20-25 minutes. Serve on their own as a lunch or supper dish or with vegetables and new potatoes for a more substantial meal.
Tip: the tomatoes should be ripe but not too soft or they will collapse in the oven. The cooking time will depend on the firmness and size of the tomatoes.