Meanwhile, lightly fry the mushrooms in the butter. Roughly chop the onion, chop the bell pepper and add both to the mushrooms. Cut a tomato into fine pieces (peel beforehand if necessary) and add to the pan. Just before it is ready (there should be a little broth left), transfer the rice into a baking dish, mix with the ingredients from the pan and the bamboo shoots and season with salt, pepper, chilli flakes and paprika.
Cut the second tomato into thin slices and distribute evenly over the rice. Cut the mozzarella into thin slices and place on top of the tomatoes. Place in the oven at 160 degrees (fan oven) until the cheese turns lightly brown.