Heat the olive oil in a large pan and sweat the onion in it. Wash, quarter and slice the zucchini, add to the onions with the garlic and fry for a while. Add the uncooked rice and fry a little too. Use the whisk to stir the tomato paste into the vegetable stock and use it to deglaze the rice. Then bring everything to a boil. Season to taste with salt, pepper and thyme and simmer for about 20 minutes on a low heat until the rice has soaked up the sauce and is done. Stir occasionally (if there is not enough liquid, just stir in a little more vegetable stock).
Just before serving, mix the diced feta cheese into the dish and let it flow a little.