Slice off the top of each tomato horizontally, reserving each tomato lid. Scoop out the seeds and heart from the middle of each and spoon into a bowl. Put the tomatoes into an ovenproof dish and drizzle with a little oil. Season well.
Heat 1 tablespoon of oil in a pan and gently cook the shallots and celery until just softened. Finely chop the tomato pulp and add to the pan. Simmer for 5 to 8 minutes until the mixture looks saucy.
Season the mixture well then stir in the rice, chilli flakes and rosemary. Pour in 200ml of the stock, cover with a lid and bring to the boil. Simmer for 15 minutes until the stock has all absorbed and the rice is just tender.
Preheat the oven to 200 C / 180 C fan / Gas 6.
Stir in the Manchego cheese, then spoon the rice mixture evenly among the tomato shells. Put the reserved tomato lids on top of each, then pour around the remaining stock. Cover the dish with foil.
Bake for 20 minutes, removing the foil for the last 5 minutes of cooking time.