Peel onion and dice finely. Heat the oil in a pot and fry the onion cubes in it until transparent. Add rice and steam briefly. Deglaze with white wine. Add stock in about 3 portions.
Cook the rice for a total of approx. 20 minutes at medium heat. Always add the next portion only when the liquid has been absorbed. Wash the tomatoes, cut off the lid with the stalk and remove the inside with a ball cutter and cut into small pieces.
Dab tomatoes dry from the inside. Whip cream until semi-stiff. Wash, clean, drain and chop the rocket. Stir butter, half of the Parmesan, tomato interior, rocket and cream into the rice. Season the risotto with salt and pepper if necessary, then fill it into the tomatoes and sprinkle with the remaining Parmesan cheese.
Place the tomatoes in an ovenproof casserole dish and bake under the grill of the oven for 3-5 minutes. Heat the stock and stir in the pesto. Remove from the oven and serve with the pesto sauce.