Peel and finely dice the onions. Wash the peppers, cut in half, core and cut into narrow strips. Wash 2 - 3 tomatoes, cut crosswise, place in hot water. Take out after 5 minutes, remove skin and cut into cubes.
Heat the butter in a saucepan, add the vegetables and stew for 20 minutes. Stir in 2 cubes of vegetable stock, yoghurt and tomato paste, season with salt and pepper.
Bring 2 liters of water to the boil, add 1 level teaspoon of salt. Add 2 sachets of rice to the water and boil for 20 minutes, then drain. Pour the sauce over the rice and serve.