Cook the rice for 15 to 20 minutes until al dente. Heat the oil and butter in a deep pan and fry the diced onions until translucent. Add the sliced mushrooms and sauté in them. Drain the canned tomatoes on a sieve and collect the juice. Measure out 125 ml. Halve the tomatoes, remove the seeds and add to the mushrooms with the juice and stock. Cook for about 12 to 15 minutes. Add rice and season with salt and pepper.
Mix with Parmesan cheese or serve separately. Serve garnished with basil leaves.