Peel and finely dice the onion and garlic. Sweat together with the rice in the oil. Add the tomatoes and the thyme leaves, deglaze with some of the stock. Do not add everything at once, as the amount of liquid in the tomatoes is different.
Cut the zucchini into small cubes. Core the tomatoes and dice the pulp as well. Mix the vegetables into the rice five minutes before the end of cooking. Season to taste with salt, pepper and sugar.
In the meantime, for the meatballs, sauté the diced onion in a little butter, do not brown. Let cool, then mix with the minced meat, egg and breadcrumbs. Add the chopped parsley, thyme leaves and lemon zest. Season to taste with salt, pepper and chilli flakes. Fry in the oil until crispy brown.