Peel and finely dice the onion and bring to the boil with the stock and a little salt. Wash the basil, dab dry and cut into fine strips, except for a little garnish. Add basil to the hot stock.
Put rice into a greased, ovenproof, round form (approx. 24 cm Ø) and pour on the stock. Cover and let it swell in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 25 minutes.
In the meantime, wash, clean and slice the zucchini and tomatoes. Drain the mozzarella and also cut into slices. Remove the mould from the oven and cover with vegetables and mozzarella in flakes.
Season with pepper. Put back into the oven and bake open for about 25 minutes. Garnish with remaining basil. (You can only cut it into pieces and lift it out with a cake lifter when it has cooled down!)