Heat the butter and oil in the cooker. Saute the onion until soft but not brown, about 2 minues. Stir in the rice, making sure to coat in thoroughly with the fat. Stir in 3 1/2 cups stock. Lock the lid in place and over high heat bring to high pressure. Adjust the heat to maintain high pressure and cook for 6 minutes. Reduce pressure with quick release method. Remove the lid carefully and taste the rice.

If it isn't sufficiently cooked add a bit more stock as you stir. Cook over medium heat until the additional liquid has been absorbed and the rice is the desired consistency, another minute or two. When the rice is ready stir in the tomatoes and mozzarella and add salt if desired. Serve immediately.